Small Molecule X-ray Facility, Department of Chemistry, Johns Hopkins University, Baltimore, MD 21218, USA

Instituto de Ciencia de Materiales de Aragón (ICMA) CSIC and Universidad de Zaragoza, Plaza San Francisco s/n, 50009 Zaragoza, Spain

Theoretical Chemistry, Zernike Institute for Advanced Materials University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Ghent Quantum Chemistry Group, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), 9000 Gent, Belgium

The tetrapyridyl ligand bbpya (bbpya = N,N-bis(2,2'-bipyrid-6-yl)amine) and its mononuclear coordination compound [Fe(bbpya)(NCS)_{2}] (**1**) were prepared. According to magnetic susceptibility, differential scanning calorimetry fitted to Sorai's domain model, and powder X-ray diffraction measurements, **1** is low-spin at room temperature, and it exhibits spin crossover (SCO) at an exceptionally high transition temperature of T_{1/2} = 418 K. Although the SCO of compound **1** spans a temperature range of more than 150 K, it is characterized by a wide (21 K) and dissymmetric hysteresis cycle, which suggests cooperativity. The crystal structure of the LS phase of compound **1** shows strong N-H···S intermolecular H-bonding interactions that explain, at least in part, the cooperative SCO behavior observed for complex **1**. DFT and CASPT2 calculations under vacuum demonstrate that the bbpya ligand generates a stronger ligand field around the iron(II) core than its analogue bapbpy (N,N'-di(pyrid-2-yl)-2,2'-bipyridine-6,6'-diamine); this stabilizes the LS state and destabilizes the HS state in **1** compared with [Fe(bapbpy)(NCS)_{2}] (**2**). Periodic DFT calculations suggest that crystal-packing effects are significant for compound **2**, in which they destabilize the HS state by about 1500 cm^{-1}. The much lower transition temperature found for the SCO of **2** compared to **1** appears to be due to the combined effects of the different ligand field strengths and crystal packing.

Towards the next generation of organic photovoltaics

Promotores: prof. dr. R. Broer

prof. dr. J.C. Hummelen

Co-promotor: dr. R.W.A. Havenith, 2016

Promotores: prof. dr. R. Broer

prof. dr. P. Th. van Duijnen

prof. dr. ir. P.H.M. van Loosdrecht, 2016

Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Laboratoire de Chimie et Physique Quantiques, IRSAMC/UMR5626, Université de Toulouse 3, Toulouse, France

Departament de Química Física I Inorgánica, Universitat Rovira i Virgili, Tarragona, Spain

Institució Catalana de Recerca i Estudis Avanats (ICREA), Barcelona, Spain

Department of Internal Medicine, Faculty of Medicine, Slovak Medical University, Limbová 12, SK-833 03 Bratislava, Slovakia

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Theoretical chemistry methods have been used to study the molecular properties of antiplatelet agents (ticlopidine, clopidogrel, prasugrel, elinogrel, ticagrelor and cangrelor) and several thiol-containing active metabolites. The geometries and energies of most stable conformers of these drugs have been computed at the Becke3LYP/6-311++G(d,p) level of density functional theory. Computed dissociation constants show that the active metabolites of prodrugs (ticlopidine, clopidogrel and prasugrel) and drugs elinogrel and cangrelor are completely ionized at pH 7.4. Both ticagrelor and its active metabolite are present at pH = 7.4 in neutral undissociated form. The thienopyridine prodrugs ticlopidine, clopidogrel and prasugrel are lipophilic and insoluble in water. Their lipophilicity is very high (about 2.5-3.5 log*P* values). The polar surface area, with regard to the structurally-heterogeneous character of these antiplatelet drugs, is from very large interval of values of 3-255 Å^{2}. Thienopyridine prodrugs, like ticlopidine, clopidogrel and prasugrel, with the lowest polar surface area (PSA) values, exhibit the largest absorption. A high value of polar surface area (PSA) of cangrelor (255 Å^{2}) results in substantial worsening of the absorption in comparison with thienopyridine drugs.

Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Ghent Quantum Chemistry Group, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), 9000 Gent, Belgium

A multidisciplinary approach involving organic synthesis and theoretical chemistry was applied to investigate a promising strategy to improve charge separation in organic photovoltaics: installing permanent dipoles in fullerene derivatives. First, a PCBM analogue with a permanent dipole in the side chain (PCBDN) and its reference analogue without a permanent dipole (PCBBz) were successfully synthesized and characterized. Second, a multiscale modeling approach was applied to investigate if a PCBDN environment around a central donor-acceptor complex indeed facilitates charge separation. Alignment of the embedding dipoles in response to charges present on the central donor-acceptor complex enhances charge separation. The good correspondence between experimentally and theoretically determined electronic and optical properties of PCBDN, PCBBz, and PCBM indicates that the theoretical analysis of the embedding effects of these molecules gives a reliable expectation for their influence on the charge separation process at a microscopic scale in a real device. This work suggests the following strategies to improve charge separation in organic photovoltaics: installing permanent dipoles in PCBM analogues and tuning the concentration of these molecules in an organic donor/acceptor blend.

Computational Biology Unit, Department of Informatics, University of Bergen, N-5020 Bergen, Norway

Institute of Biomedical Sciences, Academia Sinica, Taipei 11529, Taiwan

Theoretical Chemistry, Zernike Institute for Advanced Materials University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Ghent Quantum Chemistry Group, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), 9000 Gent, Belgium

Department of Pharmaceutical Sciences, University of Maryland School of Pharmacy, Baltimore, Maryland 21201, United States

Cation-π interactions between tyrosine amino acids and compounds containing N,N,N-trimethylethanolammonium (N(CH_{3})_{3}) are involved in the recognition of histone tails by chromodomains and in the recognition of phosphatidylcholine (PC) phospholipids by membrane-binding proteins. Yet, the lack of explicit polarization or charge transfer effects in molecular mechanics force fields raises questions about the reliability of the representation of these interactions in biomolecular simulations. Here, we investigate the nature of phenol-tetramethylammonium (TMA) interactions using quantum mechanical (QM) calculations, which we also use to evaluate the accuracy of the additive CHARMM36 and Drude polarizable force fields in modeling tyrosine-choline interactions. We show that the potential energy surface (PES) obtained using SAPT2+/aug-cc-pVDZ compares well with the large basis-set CCSD(T) PES when TMA approaches the phenol ring perpendicularly. Furthermore, the SAPT energy decomposition reveals comparable contributions from electrostatics and dispersion in phenol-TMA interactions. We then compared the SAPT2+/aug-cc-pVDZ PES obtained along various approach directions to the corresponding PES obtained with CHARMM, and we show that the force field accurately reproduces the minimum distances while the interaction energies are underestimated. The use of the Drude polarizable force field significantly improves the interaction energies but decreases the agreement on distances at energy minima. The best agreement between force field and QM PES is obtained by modifying the Lennard-Jones terms for atom pairs involved in the phenol-TMA cation-π interactions. This is further shown to improve the correlation between the occupancy of tyrosine-choline cation-π interactions obtained from molecular dynamics simulations of a bilayer-bound bacterial phospholipase and experimental affinity data of the wild-type protein and selected mutants.

Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Ghent Quantum Chemistry Group, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), B-9000 Gent, Belgium

We show that charge recombination in ordered heterojunctions depends sensitively on the degree of coherent delocalization of charges at the donor-acceptor interface. Depending on the relative sign of the electron and hole transfer integrals, such delocalization can dramatically suppress recombination through destructive quantum interference. This could explain why measured recombination rates are significantly lower than predictions based on Langevin theory for a variety of organic bulk heterojunctions. Moreover, it opens up a design strategy for photovoltaic devices with enhanced efficiencies through coherently suppressed charge recombination.

Stichting Moleculaire Quantum Mechanica, Leek, The Netherlands

Physics Institute 2, University of Cologne, Cologne, Germany

The thermal metal-insulator phase transition in the π-stacked (ED0-TIF)_{2}PF_{6} charge transfer salt is of the Peierls type. It is related to geometrical reorganisations and charge ordering phenomena. We report that dimerising displacements are involved in the mechanism of this transition. By using periodic quantum chemical calculations, we find a double well potential in which dimerisation and charge localisation become manifest. By analysing the nuclear wavefunctions we discuss the mechanism of the phase transition in terms of thermal fluctuations.

Department of Internal Medicine, Faculty of Medicine, Slovak Medical University, Limbová 12, SK-833 03 Bratislava, Slovakia

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The geometries and energies of factor Xa inhibitors edoxaban, eribaxaban, fidexaban, darexaban, letaxaban, and the dual factor Xa and thrombin inhibitors tanogitran and SAR107375 in both the gas-phase and aqueous solution were studied using the Becke3LYP/6-31++G(d,p) or Grimme's B97D/6-31++G(d,p) method. The fully optimized conformers of these anticoagulants show a characteristic L-shape structure, and the water had a remarkable effect on the equilibrium geometry. According to the calculated p*K*a values eribaxaban and letaxaban are in neutral undissociated form at pH 7.4, while fidexaban and tanogitran exist as zwitterionic structures. The lipophilicity of the inhibitors studied lies within a large range of log *P* between 1 and 4. The dual inhibitor SAR107375 represents an improvement in structural, physicochemical and pharmacokinetic characteristics over tanogitran. At blood pH, SAR107375 predominantly exists in neutral form. In contrast with tanogitran, it is better absorbed and more lipophilic and active after oral application.

Theoretical Chemistry, Zernike Institute for Advanced Materials and Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Ghent Quantum Chemistry Group, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), B-9000 Gent, Belgium

Departamento de Química, Módulo-13, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain

Instituto Madrileño de Estudios Avanzados en Nanociencias (IMDEA-Nanociencia), Cantoblanco 28049 Madrid, Spain

The fullerene-50 is a 'magic number' cage according to the 2(N + 1)2 rule. For the three lowest isomers of C_{50} with trigonal and pentagonal symmetries, we calculate the sphericity index, the spherical parentage of the occupied π-orbitals, and the current density in an applied magnetic field. The minimal energy isomer, with D_{3} symmetry, comes closest to a spherical aromat or a superaromat. In the D_{5h} bond-stretch isomers the electronic structure shows larger deviations from the ideal spherical shells, with hybridisation or even reversal of spherical parentages. It is shown that relative stabilities of fullerene cages do not correlate well with aromaticity, unlike the magnetic properties which are very sensitive indicators of spherical aromaticity. Superaromatic diamagnetism in the D_{3} cage is characterized by global diatropic currents, which encircle the whole cage. The breakdown of sphericity in the D_{5h} cages gives rise to local paratropic countercurrents.

Members of the QCMM, Alliance Ghent-Brussels, Belgium

Theoretical Chemistry, Zernike Institute for Advanced Materials and StratinghInstitute for Chemistry, University of Groningen, 9747 AG Groningen, The Netherlands

Aromatic compounds are characterised by the presence of a ring current when in a magnetic field. As a consequence, current density maps are used to assess (the degree of) aromaticity of a compound. However, often a more discrete set of so-called Nucleus Independent Chemical Shift (NICS) values is used that is derived from the current density. It is shown here that there is no simple one-to-one relationship that allows reconstructing current density maps from only NICS-values. NICS values should therefore not be used as aromaticity indices without analysis of the ab initio computed current density map.

Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam

Theoretical Chemistry, Zernike Institute for Advanced Materials and Stratingh Institute for Chemistry, University of Groningen, NL-9747 AG Groningen,The Netherlands

Ghent Quantum Chemistry Group, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), B-9000 Gent, Belgium

Department of Chemistry, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium

The planarity of small boron-based clusters is the result of an interplay between geometry, electron delocalization, covalent bonding and stability. These compounds contain two different bonding patterns involving both s and p delocalized bonds, and up to now, their aromaticity has been assigned mainly using the classical (4N + 2) electron count for both types of electrons. In the present study, we reexplored the aromatic feature of different types of planar boron-based clusters making use of the ring current approach. B^{3+/-}, B_{4}^{2-}, B_{5}^{+/-}, B_{6}, B_{7}^{-}, B_{8}^{2-}, B_{9}^{-}, B_{10}^{2-}, B_{11}^{-}, B_{12}, B_{13}^{+}, B_{14}^{2-} and B_{16}^{2-} are characterized by magnetic responses to be doubly σ and π aromatic species in which the π aromaticity can be predicted using the (4N + 2) electron count. The triply aromatic character of B_{12} and B_{13}^{+} is confirmed. The π electrons of B_{18}^{2-}, B_{19}^{-} and B_{20}^{2-} obey the disk aromaticity rule with an electronic configuration of [1σ^{2}1π^{4}1δ^{4}2σ^{2}] rather than the (4N + 2) count. The double aromaticity feature is observed for boron hydride cycles including B@B_{5}H_{5}^{+}, Li_{7}B_{5}H_{5} and M@B_{n}H_{n}^{q} clusters from both the (4N + 2) rule and ring current maps. The double π and σ aromaticity in carbon-boron planar cycles B_{7}C^{-}, B_{8}C, B_{6}C_{2}, B_{9}C^{-}, B_{8}C_{2} and B_{7}C_{3}^{-} is in conflict with the Hückel electron count. This is also the case for the ions B_{11}C_{5}^{+/-} whose ring current indicators suggest that they belong to the class of double aromaticity, in which the π electrons obey the disk aromaticity characteristics. In many clusters, the classical electron count cannot be applied, and the magnetic responses of the electron density expressed in terms of the ring current provide us with a more consistent criterion for determining their aromatic character.

Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands

First principles calculations in the framework of Density Functional Theory (DFT) and wavefunction-based correlated methods have been performed to investigate in detail the magnetic anisotropy in Sr_{3}NiPtO_{6}. This material is known for the easy-plane anisotropy with a large anisotropy constant of about 7.5-9.3 meV. We find that, by properly choosing the onsite Coulomb repulsion and exchange parameters, DFT can correctly explain the easy-plane magnetocrystalline anisotropy of the material, but the magnitude of the anisotropy constant is underestimated. On the other hand, a quantitative agreement with respect to experiments, both in magnitude and direction of the magnetic anisotropy can be recovered by using the wavefunction-based approach which is able to fully describe the multiplet physics. We also show that the presence of structural distortions of the local NiO_{6} coordination is crucial for stabilizing the magnetic anisotropy in this compound.

Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Ghent Quantum Chemistry Group, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), B-9000 Gent, Belgium

The use of larger basis sets to approach the complete basis limit, now common in quantum chemistry, is applied for the first time to a range of valence bond functions for the simplest case of molecular hydrogen. Good convergence of the energy is slow due to difficulty in getting a correct cusp near the nuclei, but it is significant. The form of the orbitals converges much faster, leading to a slight distortion of the valence bond orbitals and an enhanced overlap, irrespective whether the basis set is restricted to basis functions centred on one atom for each valence bond orbital or the full use of the basis set is allowed. This blurs the distinction between these two approaches and shows that basis set restrictions are not tenable in the complete basis set limit. Furthermore, it supports the general use of the full basis as advocated in the spin-coupled and generalised valence bond methods.

Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands

Institute for Materials Research (IMO), Hasselt University, Martelarenlaan 42, 3500, Hasselt, Belgium

Solid State and Structural Chemistry Unit, Indian Institute of Science, 560012, Bangalore, India

Ghent Quantum Chemistry Group, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), B-9000, Ghent, Belgium

IMEC, IMOMEC Associated Laboratory, Wetenschapspark 1, Diepenbeek, Belgium

Maastricht Science Programme, Maastricht University, P.O. Box 616, 6200 MD, Maastricht, The Netherlands

Current organic semiconductors for organic photovoltaics (OPV) have relative dielectric constants (relative permittivities, ε_{r} ) in the range of 2-4. As a consequence, Coulombically bound electron-hole pairs (excitons) are produced upon absorption of light, giving rise to limited power conversion efficiencies. We introduce a strategy to enhance ε_{r} of well-known donors and acceptors without breaking conjugation, degrading charge carrier mobility or altering the transport gap. The ability of ethylene glycol (EG) repeating units to rapidly reorient their dipoles with the charge redistributions in the environment was proven via density functional theory (DFT) calculations. Fullerene derivatives functionalized with triethylene glycol side chains were studied for the enhancement of ε_{r} together with poly(p-phenylene vinylene) and diketopyrrolopyrrole based polymers functionalized with similar side chains. The polymers showed a doubling of ε_{r} with respect to their reference polymers in identical backbone. Fullerene derivatives presented enhancements up to 6 compared with phenyl-C_{61}-butyric acid methyl ester (PCBM) as the reference. Importantly, the applied modifications did not affect the mobility of electrons and holes and provided excellent solubility in common organic solvents.

Theoretical ChemistryZernike Institute for Advanced Materials and Stratingh Institute for Chemistry, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281,9000 Gent, Belgium

The effect of benzo and benzocyclobutadieno annelation on the current density induced in a series of biphenylene derivatives is examined at the B3LYP/cc-pVDZ level of theory, by using the CTOCD-DZ method. Angular benzo annelation increases, whereas linear benzo annelation decreases the intensity of paratropic (antiaromatic) current density along the four-membered ring of biphenylene. The opposite effect is found for benzocyclobutadieno annelation. It is shown that the extent of local aromaticity of the four-membered ring in biphenylene congeners can vary from highly antiaromatic to nonaromatic, as a result of different modes of annelation.

Department of Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Center for Hemostasis and Thrombosis, Hemo Medika Bratislava, 851 04 Bratislava, Slovakia

The formation of the calcium and zinc salts from CaCl_{2}, ZnCl_{2} and six monomeric structural units of heparin (1-OMe ΔUA-2S, 1-OMe GlcN-S6S, 1,4-DiOMe GlcA, 1,4-DiOMe GlcN-S3S6S, 1,4-DiOMe IdoA- 2S, and 1,4-DiOMe GlcN-S6S) have been studied in gas phase and aqueous solution as model reactions for formation of heparin-Ca^{2+} and heparin-Zn^{2+} complexes. Gibbs reaction energies computed at the B3LYP/6-311++G(d,p) level of theory for the reactions studied in aqueous solution are positive and range from 20 to 250 kJ/mol.

Physics Institute 2, University of Cologne, Germany

In this paper we present a theoretical study of the nature of the ground state of the (EDO-TTF)_{2}PF_{6} charge transfer salt by using ab initio quantum chemical theory for clusters in vacuum, for embedded clusters and for the periodic system. Exemplary for other organic charge transfer systems, we show that by using a relatively low level of theory it is possible to obtain a good understanding of the electronic structure of the ground state. An assessment is made of the proximity of the triplet, the open shell singlet and the closed shell singlet states of (EDO-TTF)_{2}PF_{6}. Our calculations reveal also that several charge ordered states are very close in energy.

Promotor: prof. dr. R. Broer, 2015

Theoretical Chemistry , Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands

Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands

Ghent Quantum Chemistry Group, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), B-9000 Ghent, Belgium

Organic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands

In contrast to the equilibrium structure of the homoaromatic C_{s} homotropenylium cation, C_{8}H_{9}^{+} (**1**), which supports a pinched diatropic ring current, the C_{2v} transition state (**2**) for inversion of the methylene bridge of **1** is antiaromatic and supports a two-lobe paratropic π current, as detected by plotting B3LYP/6-31G** ipsocentric current maps. Participation of the bridge CH bonds is crucial for the change in global character of the current in the transition state, as shown by the quenching of its paratropicity on substitution of H by F. Orbital-based arguments allow rationalization of this transition between homoaromaticity and hyper(conjugative) antiaromaticity. More generally, the hyperconjugative ring current in a family of C_{2v} planar-constrained geometries of (CR_{2})C_{N-1}H_{N-1}^{q} homoannulenes (R = H, F) can be switched from paratropic (antiaromatic) to diatropic (aromatic) by variation of ring size, charge, and bridge substituent. An orbital-based counting rule accounts for these systematic trends.

Physics Institute 2, University of Cologne, Germany

The insulating and conducting phases of (EDO-TTF)_{2}PF_{6} were studied by all electron, periodic Hartree-Fock and hybrid density functional calculations. Electronic properties, such as the electronic band structure, the density of states and the Fermi surface are discussed in relation to the metal-insulator transition in this material. The nature of conduction is confirmed in both phases from their band structures and density of states. The hybrid DFT band gaps are in good agreement with experiment. Interactions are discussed on the basis of band dispersion in the inter-stack, intra-stack and inter-sheet directions. We discuss the phase transition in terms of the Peierls mechanism and our results fully support this view.

Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands

Ghent Quantum Chemistry Group, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), B-9000 Ghent, Belgium

In this paper we investigate the acoustic-to-optical up-conversion phonon processes in a multicomponent system. These processes take place during heat transport and limit the efficiency of heat ow. By combining time-resolved optical and heat capacity experiments we quantify the thermal coupling constant to be *g* ~ 0.4·10^{17} W/Km^{3}. The method is based on selective excitation of a part of a multicomponent system, and the measurement of the thermalization dynamics by probing the linear birefringence of the sample with femtosecond resolution. In particular, we study a layered multiferroic organic-inorganic hybrid, in the vicinity of the ferroelectric phase transition. A diverging term of the heat capacity is associated to soft-mode dynamics, in agreement with previous spectroscopy measurements.

Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Ghent Quantum Chemistry Group, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281, B-9000 Gent, Belgium

In organic photovoltaic devices two types of excitons can be generated for which different binding energies can be defined: the binding energy of the local exciton generated immediately after light absorption on the polymer and the binding energy of the charge-transfer exciton generated through the electron transfer from polymer to PCBM. Lowering these two binding energies is expected to improve the efficiency of the devices. Using (time-dependent) density functional theory, we studied whether a relation exists between the two different binding energies. For a series of related co-monomers, we found that the local exciton binding energy on a monomer is not directly related to that of the charge-transfer exciton on a monomer-PCBM complex because the variation in exciton binding energy depends mainly on the variation in electron affinity, which does not affect in a direct way the charge-transfer exciton binding energy. Furthermore, for the studied co-monomers and their corresponding trimers, we provide detailed information on the amount of charge transfer upon excitation and on the charge transfer excitation length. This detailed study of the excitation process reveals that the thiophene unit that links the donor and acceptor fragments of the co-monomer actively participates in the charge transfer process.

Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluis Companys 23, 08010 Barcelona, Spain

Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands

Department of Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Center for Hemostasis and Thrombosis, Hemo Medika Bratislava, 851 04 Bratislava, Slovakia

Density functional theory methods with the B3LYP and B97D functionals with triple-zeta 6-311++G(d,p) basis set have been used to study the acidity, basicity and metal affinity of L-γ-carboxyglutamic acid (GLA) and its peptide derivative [2-acetylamino-3-(methylamino)-3-oxopropyl]malonic acid (AMD-GLA). The Gibbs interaction energies of the GLA^{2-}...M^{2+} and AMD-GLA^{2-}...M^{2+} (M = Mg, Ca, Zn) complexes show an increasing binding affinity in the order Ca^{2+} < Mg^{2+} < Zn^{2+}. The transition metal Zn^{2+} is most effectively recognized by the dianions of GLA and AMD-GLA. Of the dianions studied the AMD-GLA dianion is the strongest Lewis base. Computations that include the effect of solvation showed that in water the relative stability of GLA^{2-}...M^{2+} and AMD-GLA^{2-}...M^{2+} ionic bonds is rapidly diminished. The computed interaction Gibbs energy in water is small and negative.

Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Ghent Quantum Chemistry Group, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), B-9000 Gent, Belgium

Screened Coulomb interaction in dielectrics is often used as an argument for a lower exciton binding energy and easier exciton dissociation in a high dielectric material. In this paper, we show that at length scales of excitons (10-20 Å), the screened Coulomb law is invalid and a microscopic (quantum chemical) description is necessary to describe the medium effect on exciton dissociation. The exciton dissociation energy decreases with increasing dielectric constant, albeit deviating from the inversely proportional relationship. The electronhole interaction energy, approximated with a point charge model, is apparently not affected by the dielectric constant of the environment.

Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Ghent Quantum Chemistry Group, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), B-9000 Gent, Belgium

Organic electronics form a very promising new generation of cheap, lightweight and flexible devices. Of special interest is the ability to engineer photo-physical properties of organic molecules by chemical modification. In this regard, the purpose of this research is to understand the influence of push-pull group substitution patterns on excited state properties of several donor-acceptor co-monomers and their trimers. Part of this work focuses on organic photovoltaic applications to demonstrate the practical use of the structure-property relations. In this context, the strong exciton binding energy determined by the electron-hole interaction is an important property. (Time-dependent) Density Functional Theory calculations showed a significant difference between linear- and cross-conjugated push-pull group pathways for the electron-hole interaction and the vertical exciton binding energy, which can be understood from simple Hückel theory. A linear relation between the dipole moment change upon excitation and the vertical exciton binding energy hints to a possible correlation, although this relation is less pronounced for the trimers. The overlap density between the frontier molecular orbitals alone already reveals valuable information about the relative size of the electron-hole interaction and the vertical exciton binding energy. Application of our findings in the context of organic photovoltaics results in significant support for cross-conjugated mesomeric push-pull group pathways in order to spatially separate the HOMO and LUMO.

Towards the design of new materials

Promotores: prof. dr. R. Broer

prof. dr. C. Sousa Romero, 2014

Departament de Química Física i Inorgànica, Universitat, Rovira i Virgili, Marcel·lí Domingo s/n, 43007 Tarragona, Spain

Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluis Companys 23, 08010 Barcelona, Spain

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Ghent Quantum Chemistry Group, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), B-9000 Gent, Belgium

The key parameters associated to the thermally induced spin crossover process have been calculated for a series of Fe(II) complexes with mono-, bi-, and tridentate ligands. Combination of density functional theory calculations for the geometries and for normal vibrational modes, and highly correlated wave function methods for the energies, allows us to accurately compute the entropy variation associated to the spin transition and the zero-point corrected energy difference between the low- and high-spin states. From these values, the transition temperature, T_{1/2}, is estimated for different compounds.

Stratingh Institute for Chemistry,University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Ghent Quantum Chemistry Group, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), B-9000 Gent, Belgium

We synthesized a cross-conjugated polymer containing ketones in the backbone and converted it to a linearly conjugated, cationic polyarylmethine via a process we call "spinless doping" to create a new class of materials, conjugated polyions. This process involves activating the ketones with a Lewis acid and converting them to trivalent cations via the nucleophilic addition of electron-rich aryl moieties. Spinless doping lowers the optical band gap from 3.26 to 1.55 eV while leaving the intrinsic semiconductor properties of the polymer intact. Electrochemical reduction (traditional doping) further decreases the predicted gap to 1.18 eV and introduces radicals to form positive polarons; here, n-doping produces a p-doped polymer in its metallic state. Treatment with a nucleophile (NaOMe) converts the cationic polymer to a neutral, non-conjugated state, allowing the band gap to be tuned chemically, post-polymerization. The synthesis of these materials is carried out entirely without the use of Sn or Pd and relies on scalable Friedel-Crafts chemistry.

Laboratorio de Microscopías Avanzadas (LMA), Instituto de Nanociencia de Aragón (INA) - ARAID

Departamento de Física de la Materia Condensada, Universidad de Zaragoza, 50018 Zaragoza, Spain

Transpyrenean Advanced Laboratory for Electron Microscopy (TALEM), CEMES - INA, CNRS - Universidad de Zaragoza, 30155 Toulouse, France

Department of Chemistry and CeNS, Ludwig-Maximilians-Universität München, Butenandtstrasse 5-11 (E), 81377 Munich, Germany

Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Spain

CEMES - CNRS, 30155 Toulouse, France

Universitat Rovira i Virgili, 43007 Tarragona, Spain

Institució Catalana de Recerca i Estudis Avanats (ICREA), 08010 Barcelona, Spain

Max-Planck-Institut für Eisenforschung GmbH, 40237 Düsseldorf, Germany

Groupe de Recherche en Matériaux, Microélectronique, Acoustique et Nanotechnologies (GREMAN, UMR7347), University of Tours, 37020 Tours, France

Gerencia de Investigaciòn y Aplicaciones and Instituto de Nanociencias y Nanotecnologìa, CAC-CNEA, 1650 San Martín, Argentina.

Progress in nanotechnology requires new approaches to materials synthesis that make it possible to control material functionality down to the smallest scales. An objective of materials research is to achieve enhanced control over the physical properties of materials such as ferromagnets, ferroelectrics and superconductors. In this context, complex oxides and inorganic perovskites are attractive because slight adjustments of their atomic structures can produce large physical responses and result in multiple functionalities. In addition, these materials often contain ferroelastic domains. The intrinsic symmetry breaking that takes place at the domain walls can induce properties absent from the domains themselves, such as magnetic or ferroelectric order and other functionalities, as well as coupling between them. Moreover, large domain wall densities create intense strain gradients, which can also affect the material's properties. Here we show that, owing to large local stresses, domain walls can promote the formation of unusual phases. In this sense, the domain walls can function as nanoscale chemical reactors. We synthesize a two-dimensional ferromagnetic phase at the domain walls of the orthorhombic perovskite terbiummanganite (TbMnO_{3}), which was grown in thin layers under epitaxial strain on strontiumtitanate (SrTiO_{3}) substrates. This phase is yet to be created by standard chemical routes. The density of the two-dimensional sheets can be tuned by changing the film thickness or the substrate lattice parameter (that is, the epitaxial strain), and the distance between sheets can be made as small as 5 nanometres in ultrathin films, such that the new phase at domain walls represents up to 25 per cent of the film volume. The general concept of using domain walls of epitaxial oxides to promote the formation of unusual phases may be applicable to other materials systems, thus giving access to new classes of nanoscale materials for applications in nanoelectronics and spintronics.

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Ghent Quantum Chemistry Group, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), B-9000 Gent, Belgium

Parallel Quantum Solutions, 2013 Green Acres Road, Suite A, Fayetteville, Arkansas 72703, USA

Organic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands

The homotropenylium cation (**1**, C_{8}H_{9}^{+}) is a key species in the discussion of homoaromaticity. Constrained optimisations around the minimum structure have been performed, varying the size of the gap spanned by the CH_{2}-bridge and optimising all other geometrical parameters. At each bridging distance, ab initio current-density maps have been calculated and plotted using the ipsocentric approach. Analysis of the maps, including decomposition into localised orbital contributions, gives a clear indication of a global diatropic ring current passing through the gap. The change in p_{π}-p_{π} interaction, from conventional π overlap around the conjugated seven-carbon perimeter to σ overlap (p_{σ}-p_{σ}) in the gap, results in a distinctive pinched topology, with two streams of current pinched down into one for part of the circuit. This ring current is diatropic and therefore the species **1** is aromatic on the magnetic criterion.

Ghent Quantum Chemistry Group, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), B-9000 Gent, Belgium

Providing sustainable energy is one of the biggest challenges nowadays. An attractive answer is the use of organic solar cells to capture solar energy. Recently a promising route to increase their efficiency has been suggested: developing new organic materials with a high dielectric constant. This solution focuses on lowering the coulomb attraction between electrons and holes, thereby increasing the yield of free charges. In here, we demonstrate from a theoretical point of view that incorporation of dipole moments in organic materials indeed lowers the coulomb attraction. A combination of molecular dynamics simulations for modelling the blend and ab initio quantum chemical calculations to study specific regions was performed. This approach gives predictive insight in the suitability of new materials for application in organic solar cells. In addition to all requirements that make conjugated polymers suitable for application in organic solar cells, this study demonstrates the importance of large dipole moments in polymer side-chains.

Instituto de Ciencia Molecular, Universitat de València, P. O. Box 22085, ES-46071 València, Spain

SUBATECH, UMR CNRS 6457, IN2P3/EMN Nantes/Université de Nantes, 4 Rue A. Kastler, BP 20722, 44307 Nantes Cedex 3, France

Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Ghent Quantum Chemistry Group, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), B-9000 Ghent, Belgium

In benzophenone, intersystem crossing occurs efficiently between the S_{1}(nπ^{*}) state and the T_{1} state of dominant nπ^{*} character, leading to excited triplet states after photoexcitation. The transition mechanism between S_{1}(nπ^{*}) and T_{1} is still a matter of debate, despite several experimental studies. Quantum mechanical calculations have been performed in order to assess the relative efficiencies of previously proposed mechanisms, in particular, the direct S_{1} → T_{1} and indirect S_{1} → T_{2}(ππ^{*}) → T_{1} ones. Multiconfigurational wave function based methods are used to discuss the nature of the relevant states and also to determine minimum energy paths and conical intersections. It is found that the T_{1} state has a mixed nπ^{*}/ππ^{*} character and that the T_{2}(ππ^{*}) state acts as an intermediate state between the S_{1} and T_{1} states. This result is in line with recent experiments, which suggested a two-step kinetic model to populate the phosphorescent state after photoexcitation [Aloïse et al., *J. Phys. Chem. A*, 2008, **112**, 224-231].

The ferroelectric phase transition at *T _{C}* = 340 K in (C

Center for Hemostasis and Thrombosis, Hemo Medika Bratislava, Šustekova 2, 851 04 Bratislava, Slovakia

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The methods of computational chemistry have been used to elucidate the molecular properties of coumarinic anticoagulants (acenocoumarol, phenprocoumon, warfarin and tecarfarin) and direct thrombin inhibitors (melagatran, dabigatran and their prodrug forms, ximelagatran and dabigatran etexilate). The geometries and energies of these drugs have been computed at the Becke3LYP/6-311++G(d,p) level of theory. In the case of the vitamin K antagonists (acenocoumarol, phenprocoumon, warfarin and tecarfarin), the most stable tautomer in both the gas-phase and water solution is tautomer A, which contains the 4-hydroxycoumarin moiety. The *R*(+)-enantiomer of this tautomer is the most stable structure in warfarin and acetocoumarol. For phenprocoumon, the *S*(-)-enantiomer was the most stable species. The computed dissociation constants show that these drugs are almost completely ionized at physiological pH = 7.4. Tecarfarin is the vitamin K antagonist with the highest lipophilicity. The prodrugs ximelagatran and dabigatran etexilate are described as lipophilic drugs. The prodrugs' metabolites, melagatran and dabigatran, are substantially less lipophilic. The relatively high polar surface area value of acenocoumarol (113.3) results in lessened absorption in comparison with warfarin. Phenprocoumon, with PSA value 50.4, had the highest calculated absorption of all of the anticoagulants in the study. The direct thrombin inhibitors, melagatran and dabigatran, have a high total number of proton donor and proton acceptor groups (15), a high PSA (150) and the lowest absorption of the drugs studied.

Helmholtz Zentrum Berlin für Materialien und Energie, Hahn Meitner Platz 1, 14109 Berlin, Germany

Departamento de Física, Universidade Federal do Ceará, CP 6030, CEP 60455-760 Fortaleza, CE, Brazil

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Department of Chemistry, University of South Florida, 4202 E. Fowler Ave., Tampa, FL 33620, USA

Lujan Center, LANSCE, MS H05, Los Alamos National Laboratory, Los Alamos, NM 87545, USA

Niels Bohr Institute, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark

We have investigated the dynamics in polycrystalline samples of L-methionine related to the structural transition at about 307 K by incoherent inelastic and quasielastic neutron scattering, X-ray powder diffraction as well as ab initio calculations. L-Methionine is a sulfur amino acid which can be considered a derivative of alanine with the alanine R-group CH_{3} exchanged by -CH_{3}S-(CH_{2})_{2}. Using X-ray powder diffraction we have observed at ~ 190 K an anomalous drop of the c-lattice parameter and an abrupt change of the β-monoclinic angle that could be correlated to the anomalies observed in previous specific heat measurements. Distinct changes in the quasielastic region of the neutron spectra are interpreted as being due to the onset and slowing-down of reorientational motions of the CH_{3}-S group, are clearly distinguished above 130 K in crystalline L-methionine. Large-amplitude motions observed at low frequencies are also activated above 275 K, while other well-defined vibrations are damped. The ensemble of our results suggests that the crystalline structure of L-methionine is dynamically highly disordered above 275 K, and such disorder can be linked to the flexibility of the molecular thiol-ether group.

Departament de Química Física i Inorgànica, Universitat, Rovira i Virgili, Marcel·lí Domingo s/n, 43007 Tarragona, Spain

Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluis Companys 23, 08010 Barcelona, Spain

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Institute of Theoretical and Computational Chemistry, Heinrich-Heine University Düsseldorf, Universitätsstrasse 1, 40225 Düsseldorf, Germany

Faculty of Physical Chemistry, University of Belgrade, Studentski Trg 12-16, 11158 Belgrade, Serbia

The mechanism of the light-induced spin crossover of the [Fe(bpy)_{3}]^{2+} complex (bpy=2,2'-bipyridine) has been studied by combining accurate electronic-structure calculations and time-dependent approaches to calculate intersystem-crossing rates. We investigate how the initially excited metal-to-ligand charge transfer (MLCT) singlet state deactivates to the final metastable high-spin state. Although ultrafast X-ray free-electron spectroscopy has established that the total timescale of this process is on the order of a few tenths of a picosecond, the details of the mechanisms still remain unclear. We determine all the intermediate electronic states along the pathway from low spin to high spin and give estimates for the deactivation times of the different stages. The calculations result in a total deactivation time on the same order of magnitude as the experimentally determined rate and indicate that the complex can reach the final high-spin state by means of different deactivation channels. The optically populated excited singlet state rapidly decays to a triplet state with an Fe d^{6}(t_{2g}^{5}e_{g}^{1}) configuration either directly or by means of a triplet MLCT state. This triplet ligand-field state could in principle decay directly to the final quintet state, but a much faster channel is provided by internal conversion to a lower-lying triplet state and subsequent intersystem crossing to the high-spin state. The deactivation rate to the low-spin ground state is much smaller, which is in line with the large quantum yield reported for the process.

Institute de Chimie Moléculaire et des Matériaux d'Orsay, CNRS, Université Paris Sud 11, 91405 Orsay Cedex, France

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Departament de Química Física i Inorganica, Universitat Rovira i Virgili, Marcel.lí Domingo s/n, 43007 Tarragona, Spain

CNRS, LCC (Laboratoire de Chimie de Coordination) 205, route de Narbonne, 31077 Toulouse, France

Université de Toulouse, UPS, INPT, LCC, 31077 Toulouse, France

Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluis Companys 23, 08010, Barcelona, Spain

Laboratoire National des Champs Magnétiques Intenses, UPR CNRS 3228, Université J. Fourier, 25, avenue des Martyrs, B.P. 166, 38042 Grenoble Cedex 9, France

The nature and magnitude of the magnetic anisotropy of heptacoordinate mononuclear Ni^{II} and Co^{II} complexes were investigated by a combination of experiment and ab initio calculations. The zero-field splitting (ZFS) parameters *D* of [Ni(H_{2}DAPBH)-(H_{2}O)_{2}](NO_{3})_{2}·2H_{2}O (**1**) and [Co-(H_{2}DAPBH)(H_{2}O)(NO_{3})](NO_{3}) [**2**; H_{2}DAPBH=2,6-diacetylpyridine bis-(benzoyl hydrazone)] were determined by means of magnetization measurements and high-field high-frequency EPR spectroscopy. The negative *D* value, and hence an easy axis of magnetization, found for the Ni^{II} complex indicates stabilization of the highest M_{S} value of the S=1 ground spin state, while a large and positive *D* value, and hence an easy plane of magnetization, found for Co^{II} indicates stabilization of the M_{S}= ±1/2 sublevels of the S=3/2 spin state. Ab initio calculations were performed to rationalize the magnitude and the sign of *D*, by elucidating the chemical parameters that govern the magnitude of the anisotropy in these complexes. The negative *D* value for the Ni^{II} complex is due largely to a first excited triplet state that is close in energy to the ground state. This relatively small energy gap between the ground and the first excited state is the result of a small energy difference between the d_{xy} and d_{x2-y2} orbitals owing to the pseudo-pentagonal-bipyramidal symmetry of the complex. For Co^{II}, all of the excited states contribute to a positive *D* value, which accounts for the large magnitude of the anisotropy for this complex.

Center for Hemostasis and Thrombosis, Hemo Medika Bratislava, 851 04 Bratislava, Slovakia

Department of Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Density functional theory methods with the B3LYP functional have been used to letter the acidity of carboxyl, O-sulfo and N-sulfo groups in six basic monomeric structural units of heparin (1-OMe ΔUA-2S, 1- OMe GlcN-S6S, 1,4-DiOMe GlcA, 1,4-DiOMe GlcN-S3S6S, 1,4-DiOMe IdoA-2S, and 1,4-DiOMe GlcN-S6S). The predicted gas-phase acidity of the acidic functional groups in the monomeric structural units of heparin is: O-sulfo > N-sulfo > carboxyl. The computed pK_{a} values provide the same order of acidity as was observed in water solution. This implies that hydration does not change ordering of acidity of major acidic groups of monomeric structural units of heparin.

Departament de Química Física i Inorgànica, Universitat, Rovira i Virgili, Marcel·lí Domingo s/n, 43007 Tarragona, Spain

Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluis Companys 23, 08010 Barcelona, Spain

Departament de Química Física and Institut de Química Teòrica i Computacional, Universitat de Barcelona, C/Martí i Franquès 1, 08028 Barcelona, Spain.

The spin crossover behavior of the two [Fe(mtz)_{6}]^{2+} complexes occupying different lattice sites in Fe(mtz)_{6}(BF_{4})_{2} is addressed by combining quantum chemical calculations with a careful analysis of the crystal structure. It is first established from the calculations that the energy difference between high spin and low spin states depends on the orientation of the tetrazole ligands; small rotation angles favor the low spin state, while for angles larger than ~20° the high spin state is more stable. The crystal structure shows that the two complexes have different average rotation angles of the ligands. It is larger for the site that remains HS down to low temperatures and smaller for the site that shows spin crossover to LS. The origin of the different rotation angles is found to be determined by a subtle interplay amongst steric repulsion between the ligands, H···F interactions between the complex and the counterions, and intersite interactions involving N···H contacts and π-π interactions between the N=N double bonds of the tetrazole rings.

Promotor: prof. dr. A. Meijerink

Co-promotor: dr. R.W.A. Havenith, 2013

Promotores: prof. dr. R. Broer

prof. dr. C. de Graaf, 2013

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The B_{20}^{2-} cluster is predicted to exhibit a planar sheet-like structure with a circular circumference. Orbital plots and energy correlations demonstrate the close correspondence between the electronic structure of B_{20}^{2-} and the Bessel functions describing the waves of a quantum mechanical particle conned to a disk. The π-band of B_{20}^{2-}, and its B_{19}^{-} congener, contains 12 π-electrons, forming a (1σ)^{2}(1π)^{4}(1δ)^{4}(2σ)^{2} configuration, which corresponds to a "disk aromaticity" electron count. The analogy not only applies to the π-band, but also extends to the 50 valence σ-electrons. The occupied σ-orbitals are assigned on the basis of radial and angular nodes of the scalar disk waves. The magnetic response of the cluster was examined by Nucleus Independent Chemical Shift (NICS) values and current density calculations based on the ipsocentric model. B_{20}^{2-} is found to exhibit a remarkable inner paratropic current in the σ-channel and an outer diatropic current in the π-channel. The orbital excitations responsible for the antiaromaticity in σ and the disk-aromaticity in π are identified.

Department of Inorganic Chemistry, Faculty of Science, Charles University in Prague, Hlavova 2030, CZ-12840 Prague, Czech Republic

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen Nijenborgh 4, 9747 AG Groningen, The Netherlands

Eenheid Algemene Chemie (ALGC), Vrije Universiteit Brussel (VUB), Pleinlaan 2, B-1050 Brussels, Belgium

Reaction of organoantimony and organobismuth oxides (LSbO)_{2} and (LBiO)_{2} (where L is [2,6-bis(dimethylamino)methyl]phenyl) with four equivalents of the organoboronic acids gave new heteroboroxines LM[(OBR)_{2}O] **1a** **2c** (for M = Sb: R = Ph (**1a**), 4-CF_{3}C_{6}H_{4} (**1b**), ferrocenyl (**1c**); for M = Bi: R = Ph (**2a**), 4-CF_{3}C_{6}H_{4} (**2b**), and ferrocenyl (**2c**)). Analogously, reaction between organotin carbonate L(Ph)Sn(CO_{3}) and two equivalents of organoboronic acids yielded compounds L(Ph)Sn[(OBR)_{2}O] (where R = Ph (**3a**), 4-CF_{3}C_{6}H_{4} (**3b**), and ferrocenyl (**3c**)). All compounds were characterized by elemental analysis and NMR spectroscopy. Their structure was described both in solution (NMR studies) and in the solid state (X-ray diffraction analyses **1a**, **1c**, **2b**, **3b**, and **3c**). All compounds contain a central MB_{2}O_{3} core (M = Sb, Bi, Sn), and the bonding situation within these rings and their potential aromaticity was investigated by the help of computational methods.

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Departament de Química Física i Inorganica, Universitat Rovira i Virgili, Marcel.lí Domingo s/n, 43007 Tarragona, Spain

Université Paris Sud 11, CNRS, Institute de Chimie Moléculaire et des Matériaux d'Orsay, F-91405 Orsay, France

Laboratoire National des Champs Magnétiques Intenses, UPR CNRS 3228, Université J. Fourier, 25, avenue des Martyrs, B.P. 166, 38042 Grenoble Cedex 9, France

National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, United States

Department of Physics, Florida State University, Tallahassee, Florida 32306, United States

Université de Lorraine, CRM2, UMR 7036, Boulevard des Aiguillettes, Vandoeuvre les Nancy, F-54506, France

CRNS, CRM2, UMR 7036, Boulevard des Aiguillettes, Vandoeuvre les Nancy, F-54506, France

This paper reports the experimental and theoretical investigations of two trigonal bipyramidal Ni(II) complexes, [Ni(Me_{6}tren)Cl]-(ClO_{4}) (**1**) and [Ni(Me_{6}tren)Br](Br) (**2**). High-field, high-frequency electron paramagnetic resonance spectroscopy performed on a single crystal of **1** shows a giant uniaxial magnetic anisotropy with an experimental D_{expt} value (energy difference between the M_{s} = ± 1 and M_{s} = 0 components of the ground spin state S = 1) estimated to be between -120 and -180 cm^{-1}. The theoretical study shows that, for an ideally trigonal Ni(II) complex, the orbital degeneracy leads to a first-order spin-orbit coupling that results in a splitting of the M_{s} = ± 1 and M_{s} = 0 components of approximately -600 cm^{-1}. Despite the Jahn-Teller distortion that removes the ground term degeneracy and reduces the effects of the first-order spin-orbit interaction, the D value remains very large. A good agreement between theoretical and experimental results (theoretical D_{theor} between -100 and -200 cm^{-1}) is obtained.

Zernike Institute for Advanced Materials, Rijksuniversiteit Groningen, Nijenborgh 4, NL-9747 AG Groningen, The Netherlands

Department of Computational Materials Physics, University of Vienna, Sensengasse 8/12, A-1060 Wien, Austria

Two finite-field implementations for the calculation of chemical shieldings of molecular systems using a plane-wave basis set and the Gauge-Including Projector-Augmented-Wave method are presented. The direct approach mimics the nuclear magnetic resonance experiment in that it puts the molecule in a uniform magnetic field and obtains shieldings from the current response. The other is based on the recently introduced "converse method" [T. Thonhauser, D. Ceresoli, A. A. Mostofi et al., J. Chem. Phys. **131**, 101101 (2009)]. In both methods two-center contributions to the shieldings can be included via a numerically simple augmentation construction. Results obtained with both methods are discussed as well as (dis)similarities in their behaviors.

Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands

Debye Institute for Nanomaterials Science, Organic Chemistry and Catalysis, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Single crystal X-ray diffraction reveals that 4,4'-bis(tetrahydrothiopyranyl) crystallizes in an equatorial-equatorial geometry with a gauche conformation along the central carbon-carbon bond. B3LYP/6-311G and MP2/6-311G calculations show that the antiperiplanar conformation is higher in energy than the gauche one because of sulfur induced stretching and widening of the cyclohexane-like rings. Calculations at various levels of theory suggest that in the antiperiplanar region the twisting coordinate of 4,4'-bis(tetrahydrothiopyranyl) exhibits a very shallow double-well potential. The gauche molecular structure of 4,4'-bis(tetrahydrothiopyranyl) thwarts efficient packing of its molecules in the solid state.

Department of Chemistry, Quy Nhon University, Quy Nhon, Vietnam

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Unimolecular decompositions of neutral (NH_{2}CHO) and protonated (NH_{3}CHO^{+}) formamide, an active precursor of biomolecules in prebiotic chemistry, are investigated in the ground (S_{0}) and first triplet (T_{1}) and singlet (S_{1}) excited states. Different decomposition channels including the homolytic bond dissociations, dehydration, decarbonylation, dehydrogenation, etc., are explored using coupled-cluster theory (CCSD(T)/CBS method) for both S_{0} and T_{1} states and RASPT2(18,15)/6-31G(d,p) computations for the S_{1} state. On S_{1} and T_{1} energy surfaces, formamide preferentially follows C-N homolytic bond cleavages forming NH_{2} + HCO radical pairs. Formation of HCN and HNC from dehydration of neutral and protonated formamide via formimic acid and aminohydroxymethylene isomers has higher energy barriers. A strong stabilization upon triplet excitation of the two latter isomers significantly facilitates the interconversions between isomers, and thus considerably reduces the energy barriers for dehydration pathways. The most probable pathways for HCN and HNC generation are found to be dehydration of formamide in the T_{1} state. Dehydration pathways from the neutral S_{1} and protonated T_{1} forms lead to stable complexes of HCN and HNC with water but are associated with large energy barriers. Overall, in the lower-lying excited states of either neutral or protonated formamide, dehydration is not competitive with homolytic C-N bond cleavages, which finally lead to formation of CO.

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Dipartimento di Scienze Chmimiche e Farmaceutiche, Università di Trieste, Via L. Giorgieri 1, I-34127 Trieste, Italy

Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali, INSTM, Unita' di Trieste, I-34127 Trieste, Italy

INFM DEMOCRITOS National Simulation Center, Trieste, Italy

The surface plasmon resonance has been theoretically investigated in gold nanowires by means of time-dependent density functional theory. Linear chains of Au atoms and nanowires with the structure of the fcc bulk gold grown along the <110> and <111> directions have been considered. The effects of changing the length and the section on the plasmon have been studied. Strong photoabsorption is found when the length is above 2 nm: in that case the absorption profile is characterized by a sharp peak, and its analysis reveals that many configurations contribute to the transition, confirming its collective nature as an s ← s intraband transition. As expected, the effect of increasing the length is reflected in a red shift of the plasmon.

Institute of Mathematical & Physical Sciences, Prifysgol Aberystwyth University, Wales SY23 3BZ, UK

Department of Physics, Indian Institute of Technology Bombay, Powai Mumbai, India

School of Mechanical & Advanced Materials Engineering, Ulsan National Institute of Science & Technology, UNIST-gil 50, Ulsan 689-805, Republic of Korea

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Luoyu Road, Wuhan 430074, China

Radboud University Nijmegen, Institute for Molecules and Materials, Heyendaalseweg 135, 6525 AJ Nijmegen,The Netherlands

A study of the organic semiconductor F8TBT is presented, directly comparing a conventional form (F8TBT-out) with a form with varied alkyl side-chain position (F8TBT-in), in terms of optical properties and device performance in light-emitting-diodes (LEDs). Computational simulations of the side-chain position with respect to the TBT unit reveal geometrical differences between F8TBT-out and F8TBT-in. π-π conjugation on the backbone of F8TBT-in is interrupted by a distortion of the benzothiadiazole ring, leading to a blueshift of the absorption spectrum and increased photoluminescence quantum efficiency. Both conventional and hybrid LEDs demonstrate that devices with F8TBT-in show improved performance, as compared to F8TBT-out, illustrating how tuning the optoelectronic properties of conjugated polymers by varying the placement of side chains has an important role in device optimization.

Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

We provide a short review of modern 'plastic' solar cells, a broad topic that spans materials science, physics, and chemistry. The aim of this review is to provide a primer for non-experts or researchers in related fields who are curious about this rapidly growing field of interdisciplinary research. We introduce the basic concepts of plastic solar cells and design rules for maximizing their efficiency, including modern quantum chemical calculations that can aide in the design of new materials. We discuss the history of the materials and modern trends in polymeric donor materials and fullerene acceptors, and provide demonstrative data from hybrid polymer/quantum dot devices.

Department of Theoretical and Structural Chemistry, University of Łódź, Tamka 12, 91-403 Łódź, Poland

Department of Chemistry, University of Warsaw, 02 093 Warsaw, Poland

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Department of Chemistry, University of Sheffield, Sheffield, S3 7HF, UK.

The ring-current aromaticity of the bicalicene molecule arises, in spite of the 16 π carbon perimeter, from strong local diatropic circulations on the two pentagonal rings, as shown by current-density maps computed at the ipsocentric RHF/6-311G** and DFT/6-311G** levels of theory. Conjugated-circuit models cannot capture this pattern of circulation as it arises from 'ionic' contributions in a valence-bond picture. Canonical molecular-orbital analysis reveals a cancellation of paratropic and diatropic frontier-orbital contributions, which explains the difficulties that Hückel-based models have in producing qualitatively correct current-density maps for this molecule. Other measures of aromaticity reflect, to different extents, the dominance of the 'tetraionic' contribution to the aromaticity of this species.

Department of Chemistry, Brown University, Providence, RI, USA

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Understanding the geometry, energetics and dynamics of solvated transition metal (TM) compounds is decisive in characterizing and optimizing their function. Here, we demonstrate that it is possible to quantify the structural dynamics of solvated [Ru^{II}(bpy)_{3}], an important TM-complex for solar-energy harvesting research, by using state-of-the art force fields together with molecular simulations. Electronic excitation to [Ru^{II}(bpy)_{3}] leads to a nonequilibrium system in which excess energy is redistributed to the surrounding solvent following a cascade of dynamical effects that can be characterized by the simulations. The study reveals that the structure of the surrounding solvent relaxes towards the equilibrium on a sub-picosecond to a few-picosecond time scale. Analysis of solvent residence and rotational reorientation times during relaxation demonstrates increased dynamics in the inner solvation sphere on the picosecond time scale. Energy transfer to the solvent occurs on different time scales for the different degrees of freedom which range from a few hundred fs to several picoseconds.

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Department of Physics, Technical University Dresden, 01062 Dresden, Germany

By many-body quantum-chemical calculations, we investigate the role of two structural effects - local ligand distortions and the anisotropic Cd-ion coordination - on the magnetic state of Cd_{2}Os_{2}O_{7}, a spin S=3/2 pyrochlore. We find that these effects strongly compete, rendering the magnetic interactions and ordering crucially dependent on these geometrical features. Without trigonal distortions, a large easy-plane magnetic anisotropy develops. Their presence, however, reverses the sign of the zero-field splitting and causes a large easy-axis anisotropy (D≈-6.8 meV), which in conjunction with the antiferromagnetic exchange interaction (J≈6.4 meV) stabilizes an all-in-all-out magnetic order. The competition uncovered here is a generic feature of pyrochlore magnets.

Center for Hemostasis and Thrombosis, Hemo Medika, Bratislava, 851 04, Bratislava, Slovakia

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Our work reports in detail the results of systematic large-scale theoretical investigations of the complexes modeling heparin-protein interaction (CH_{3}OSO_{3}^{-}···Arg^{+}, CH_{3}NHSO_{3}^{-}···Arg^{+}, CH_{3}CO_{2}^{-}···Arg^{+}, CH_{3}OSO_{3}^{-}···Lys^{+}, CH_{3}NHSO_{3}^{-}···Lys^{+}, CH_{3}CO_{2}^{-}···Lys^{+}, CH_{3}OPO_{3}H^{2-}···Arg^{+}, CH_{3}OPO_{3}H^{2-}···Lys^{+}, CH_{3}O(CH_{3})PO_{2}^{-}···Arg^{+}, CH_{3}O(CH_{3})PO_{2}^{-}···Lys^{+}, 1,4-DiOMeIdoA2SNa^{-}···Arg^{+}, 1,4-DiOMeIdoA2SNa^{-}···Lys^{+}) using Becke3LYP and B97D levels of the density functional theory, as well as at MP2 *ab initio* method. Although initial geometries of complexes paired anionic and cationic species (ionic hydrogen bonds), full geometry optimization of isolated systems resulted in several cases with relaxed geometry and complexes stabilized *via* neutral hydrogen bonds. Hydration caused appreciable geometry changes, especially for substituents (carboxylate and sulphate groups) of the saccharidic part of the complex. The computed Gibbs energies ΔG° of the ionic hydrogen bond systems are negative and high (from -340 to -450 kJ mol^{-1}). In complexes with neutral H-bonds the large destabilizing effect of entropy drives the association reaction to the left. However, owing to a sufficient enthalpy change Gibbs energies are indeed negative, but small (from -20 to 0 kJ mol^{-1}) and the tendency to associate in gas-phase for the complex CH_{3}OPO_{3}H^{-}···Lys^{+} is negligible. The phosphate anion in its complexes with arginine and lysine proved the lowest tendency to associate. Displacing of Na^{+} ions from heparine binding sites by protonated arginine and lysine molecules resulted in positive reaction energies. Solvent (water) reversed the reactivity. Reaction energies computed for the reactions conducted in water are calculated negative, *i.e.* water drives these reactions to the right.

Center for Hemostasis and Thrombosis, Hemo Medika Bratislava, Šustekova 2, 851 04 Bratislava, Slovakia

Department of Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The effects of complexation by Li^{+}, Na^{+}, K^{+}, Mg^{2+}, and Ca^{2+} counterions and water on the molecular structure of the Fondaparinux pentameter (D-E-F-G-H) and its dimer units (D-E, E-F, F-G and G-H) are studied using Becke 3LYP hybrid density functional theory and molecular modeling. The ionic charge state, the number of metal ion adducts and the counterion radii are important factors that influence counterion-induced conformational changes in these pentamers and dimers of heparin. The displacement of the Li^{+}, Na^{+}, K^{+}, Mg^{2+} and Ca^{2+} cations from their binding sites in the salts results in appreciable changes in the anion conformations. The interaction energies are very negative and span a broad range from 21900 to 216000 kJ mol^{-1}, which is the result of multiple coordinate bonds between ions and basic centers in glycosaminoglycan units.

Forschungszentrum Jülich, Institut of Complex Systems, 52425 Jülich, Germany

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark

We present the Raman spectra of L-methionine (C_{5}H_{11}NO_{2}S) monocrystals obtained in the spectral region ranging from 3200 to 50 cm^{-1} at temperatures from 20 to 375 K. We investigated the dynamics of the different functional groups in L-methionine and related their behaviour to the structural transition previously reported at about 307 K. Additionally, on cooling, changes in the intensities of some Raman bands were associated with conformational changes of at least one of the two L-methionine conformers in the monoclinic unit cell in the interval 160-140 K. Thermal analysis and DFT calculations provide further support to the interpretation of the Raman results.

Université de Toulouse, UPS, INPT, LCC, 31077 Toulouse (France)

Laboratoire de Chimie et de Physique Quantique, IRSAMC/UMR5626, Université de Toulouse III, 118 route de Narbonne, 31062 Toulouse Cedex 4 (France)

Departament de Química Física i Inorganica, Universitat Rovira i Virgili, Marcel.lí Domingo s/n, 43007 Tarragona (Spain)

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Two novel mononuclear five-coordinate nickel complexes with distorted square-pyramidal geometries are presented. They result from association of a tridentate "half-unit" ligand and 6,6'-dimethyl-2,2'-bipyridine according to a stepwise process that highlights the advantage of coordination chemistry in isolating an unstable tridentate ligand by nickel chelation. Their zero-field splittings (ZFS) were studied by means of magnetic data and state-of-the-art ab initio calculations. Good agreement between the experimental and theoretical axial D parameters confirms that large single-ion nickel anisotropies are accessible. The synthetic process can also yield dinuclear nickel complexes in which the nickel ions are hexacoordinate. This possibility is facilitated by the presence of phenoxo oxygen atoms in the tridentate ligand that can introduce a bridge between the two nickel ions. Two different double bridges are characterized, with the bridging oxygen atoms coming from each nickel ion or from the same nickel ion. This coordination change introduces a difference in the antiferromagnetic interaction parameter J. Although the magnetic data confirm the presence of single-ion anisotropies in these complexes, these terms cannot be determined in a straightforward way from experiment due to the mismatch between the principal axes of the local anisotropies and the presence of intersite anisotropies.

A method for the calculation of X-ray photoelectron spectra (XPS) based on the use of the normalized elimination of the small component (NESC) formalism combined with the restricted active space state interaction (RASSI) approach with atomic mean field integrals (AMFI) is developed. Benchmark calculations carried out for the 4f XPS of U^{5+} show that the NESC/RASSI/AMFI method is capable of reproducing the results of the full 4-component relativistic calculations with excellent accuracy. The NESC/RASSI/ AMFI method is applied to study the 4p and 5p XPS of ytterbium phosphide YbP. The results of the calculations suggest an alternative interpretation of the satellite peaks in the 4p XPS of YbP.

Promotores: prof. dr. R. Broer and prof. dr. A. Meijerink, 2012

Limits, Models, Consequences

Promotores: prof. dr. T.A.F. Kuipers and prof. dr. R. Broer, 2012

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

We comment on the paper [Song et al., J. Comput. Chem. 2009, 30, 399] and discuss the efficiency of the orbital optimization and gradient evaluation in the Valence Bond Self Consistent Field (VBSCF) method. We note that Song et al. neglect to properly reference Broer et al., who published an algorithm [Broer and Nieuwpoort, Theor. Chim. Acta 1988, 73, 405] to use a Fock matrix to compute a matrix element between two different determinants, which can be used for an orbital optimization. Further, Song et al. publish a misleading comparison with our VBSCF algorithm [Dijkstra and van Lenthe, J. Chem. Phys. 2000, 113, 2100; van Lenthe et al., Mol. Phys. 1991, 73, 1159] to enable them to favorably compare their algorithm with ours. We give detailed timings in terms of different orbital types in the calculation and actual timings for the example cases.

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Resonance energy is one of the criteria to measure aromaticity. The effect of the use of different orbital models is investigated in the calculated resonance energies of cyclic conjugated hydrocarbons within the framework of the ab initio Valence Bond Self-Consistent Field (VBSCF) method. The VB wave function for each system was constructed using a linear combination of the VB structures (spin functions), which closely resemble the Kekulé valence structures, and two types of orbitals, that is, strictly atomic (local) and delocalized atomic (delocal) p-orbitals, were used to describe the π-system. It is found that the Pauling-Wheland's resonance energy with nonorthogonal structures decreases, while the same with orthogonalized structures and the total mean resonance energy (the sum of the weighted off-diagonal contributions in the Hamiltonian matrix of orthogonalized structures) increase when delocal orbitals are used as compared to local p-orbitals. Analysis of the interactions between the different structures of a system shows that the resonance in the 6π electrons conjugated circuits have the largest contributions to the resonance energy. The VBSCF calculations also show that the extra stability of phenanthrene, a kinked benzenoid, as compared to its linear counterpart, anthracene, is a consequence of the resonance in the π-system rather than the H-H interaction in the bay region as suggested previously. Finally, the empirical parameters for the resonance interactions between different 4n+2 or 4n π electrons conjugated circuits, used in Randic's conjugated circuits theory or Herdon's semi-emprical VB approach, are quantified. These parameters have to be scaled by the structure coefficients (weights) of the contributing structures.

The geometry of ethylenedioxy-tetrathiafulvalene, EDO-TTF, plays an important role in the metal-insulator transition in the charge transfer salt (EDO-TTF)_{2}PF_{6}. The planar and off-planar geometrical conformations of the EDO-TTF molecules are explained using an extended Debye polarizability model for the bond angle. The geometrical structure of EDO-TTF is dictated by its four sulfur bond angles and these are, in turn, determined by the polarizability of the sulfur atoms. With Hartree-Fock and second-order Møller-Plesset perturbation theory calculations on EDO-TTF, TTF, H_{2}S, and their oxygen and selenium substituted counterparts we confirm this hypothesis. The Debye polarizability model for bond angles relates directly the optimum bond angle with the polarizability of the center atom. Considering the (EDO-TTF)_{2}PF_{6} material in this light proves to be very fruitful.

Different ab initio methods, namely multi-reference and nonorthogonal configuration interaction techniques, are explored for their applicability in studying the singlet fission problem. It has been shown for 2-methyl-1,5-hexadiene that the ^{1}TT state can be identified using multi-reference techniques. The geometrical and vibrational properties of the ^{1}TT state are such that they can be approximated with those of the ^{5}TT state. A proof of principle is given for the calculation of the singlet fission pathway driven by nuclear motion: efficient singlet fission can take place if the ^{1}TT and S_{1} states are close in energy with a large non-adiabatic coupling matrix element at the S_{1} geometry, and the energy of the S_{0} state is well below that of the ^{1}TT state at the ^{1}TT geometry. The nonorthogonal configuration interaction method was used to treat a tetracene trimer. It has been shown that the first excited states can be interpreted as delocalised states; interaction with charge-transfer base states plays an important role. The ^{1}TT states are localised on one pair of molecules. The electronic coupling between the diabatic S[n] and ^{1}TT[m] states is in the meV range, confirming previous estimates. The charge-transfer base states enhance the coupling between the S[1]/S[2] and ^{1}TT[2] excited states.

Laboratoire de Chimie et Physique Quantiques, Université de Toulouse 3, 118, route de Narbonne, 31062 Toulouse, France

Departament de Quimica Fisica i Inorganica, Universitat Rovira i Virgili, Marcel-lí Domingo s/n, 43007 Tarragona, Spain

Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluis Companys 23, 08010 Barcelona, Spain

Cupric Oxide (CuO) has been described to belong to the quasi one-dimensional antiferromagnetic compounds. It has also been suggested that cupric oxide possesses strong magnetic anisotropy, which is possibly related to the observed ferroelectricity in this material. In this paper, the magnetic interactions of CuO are investigated using the embedded cluster approach. Accurate wave-function based methods have been employed to describe the interactions along all copper-oxygen chain directions. Both two-center and three-center clusters are considered in our calculations. The antisymmetric anisotropic interaction parameters are also calculated for the two-center clusters by applying an effective Hamiltonian theory. Our results show that the magnetic interactions are dominated by the antiferromagnetic coupling between copper ions along the [101] direction with a *J* value of 382 cm^{-1}, in good agreement with experiment. The results for the interplane magnetic interactions reveal competition between nearest neighbor ferromagnetic coupling and second nearest neighbor antiferromagnetic interaction. We also nd non-negligible Dzyaloshinskii-Moriya interaction in the *ac*-plane of CuO.

Eenheid Algemene Chemie (ALGC), Vrije Universiteit Brussel (VUB), Faculteit Wetenschappen, Pleinlaan 2, 1050 Brussels, Belgium

Department of Organic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands

Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK

Ring currents calculated in the ipsocentric CTOCD-DZ formalism are presented for four representative metallabenzenes, compounds in which a benzene CH group is formally replaced by a transition metal atom with ligands. Aromaticity is probed using ring currents computed using non-relativistic and relativistic orbitals (derived with relativistic effective core potentials or ZORA). Maps computed at different levels of relativistic theory turn out to be similar, showing that orbital nodal character is the main determinant of ring current. Diatropic/paratropic global ring currents in these compounds, and also circulations localised on the metal centre, are interpreted in terms of contributions of localised π-type orbitals and metal d-orbitals, respectively. All four considered metallabenzenes should be regarded as 6π electron species, despite the fact that three support diatropic ('aromatic') ring currents and one a paratropic ('anti-aromatic') current. The current-density maps determine the correct way to count electrons in these species: differential occupation of d-orbitals of formal π-symmetry contributes to circulation on the metal centre, but not around the benzenoid ring. The overall trend from strongly diatropic to weakly paratropic ring currents along the series 1 to 4 is explained by the increasing strength of interaction between formally non-bonding orbitals on the metal centre and C_{5}H_{5} moiety, which together make up the six-membered ring.

Institut de Química Computacional, Universitat de Girona, Campus Montilivi, 17071 Girona, Catalonia, Spain

Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluís Companys 23, 08010 Barcelona, Spain

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

We developed a procedure that combines three complementary computational methodologies to improve the theoretical description of the electronic structure of nickel oxide. The starting point is a Car-Parrinello molecular dynamics simulation to incorporate vibrorotational degrees of freedom into the material model. By means of complete active space self-consistent field second-order perturbation theory (CASPT2) calculations on embedded clusters extracted from the resulting trajectory, we describe localized spectroscopic phenomena on NiO with an efficient treatment of electron correlation. The inclusion of thermal motion into the theoretical description allows us to study electronic transitions that, otherwise, would be dipole forbidden in the ideal structure and results in a natural reproduction of the band broadening. Moreover, we improved the embedded cluster model by incorporating self-consistently at the complete active space self-consistent eld (CASSCF) level a discrete (or direct) reaction field (DRF) in the cluster surroundings. The DRF approach offers an efficient treatment of electric response effects of the crystalline embedding to the electronic transitions localized in the cluster. We offer accurate theoretical estimates of the absorption spectrum and the density of states around the Fermi level of NiO, and a comprehensive explanation of the source of the broadening and the relaxation of the charge transfer states due to the adaptation of the environment.

Departament de Química Física i Inorganica, Universitat Rovira i Virgili, Marcel.lí Domingo s/n, 43007 Tarragona, Spain

Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluis Companys 23, 08010 Barcelona, Spain

The possible origin of the spiral spin structure in multiferroic LiCu_{2}O_{2} is studied by calculating all relevant isotropic and anisotropic magnetic interactions in the material. The coupling constants are extracted from accurate ab initio quantum chemical calculations with an effective Hamiltonian theory. First, the anisotropic or Dzyaloshinskii-Moriya interactions are found to be negligible. Secondly, we obtain small isotropic interactions of the spin moments located on different chains, which classifies the material as a quasi-one-dimensional magnetic system. The intrachain isotropic interactions between nearest neighbors are relatively large and ferromagnetic, while second-neighbor interactions along the chain have antiferromagnetic character and are about half the magnitude of the former. This frustration leads to a spiral spin structure, which can be subjected to electric polarization.

Department of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The concept of bond ionicity, obtained via a valence bond analysis, is invoked in the interpretation of the catalytic activity of supported vanadium oxides, in analogy with previous work conducted within the framework of conceptual DFT. For a set of model clusters representing the vanadium oxide supported on SiO_{2}, Al_{2}O_{3}, TiO_{2}, ZrO_{2}, the ionic character of the vanadium-oxygen bond, involved in the dissociative adsorption of methanol on the catalyst, was quantified. Detailed scrutiny shows that this ionicity increases from the Al through the Zr support, in agreement with the increasing catalytic activity through this series; the case of the Si supported oxide is found to be an exception however, giving rise to the most ionic V-O bond of the different compounds studied. This finding is confirmed by calculations on smaller clusters focusing on detail in the π back bonding.

Promotor: prof. dr. M. Filatov, 2011

Fifty years ago Rudolf L. Mõssbauer discovered the recoilless nuclear resonance absorption of γ-rays while working on his doctoral thesis. This phenomenon, which rapidly developed into a new spectroscopic technique is known as Mõssbauer effect. Over the last couple of decades, Mõssbauer spectroscopy has become one of the most captivating tools in chemical physics providing information about the chemical environment of the resonating nucleus on an atomic scale. The most well-known application is the determination of iron ^{57}Fe in crystalline and in disordered solid samples. Besides iron, there are many elements in the periodic table which have Mõssbauer active nuclei.

Promotor: prof. dr. M. Filatov, 2011

The immense progress in science and technology nowadays calls for new doorways to developing memory storage devices, processing units and, importantly, entirely new means of turning light energy into useful work. The heritage of nature offers us an abundance of smart solutions realized at molecular level: biologically active molecules are driven by heat, electron and proton transfer, cis-trans isomerizations. The reactions involved are generally very fast and efficient, proceeding often on picosecond and subpicosecond timescales. It becomes clear that a further advance in technology should inevitably adopt the mechanisms involved in the molecules and macromolecules in living bodies and plants.

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK

One-electron oxidation of the non-alternant polycyclic aromatic hydrocarbon pleiadiene and related cyclohepta[c,d]pyrene and cyclohepta[c,d]fluoranthene in THF produces corresponding radical cations detectable in the temperature range of 293-263 K only on the subsecond time scale of cyclic voltammetry. Although the EPR-active red-coloured pleiadiene radical cation is stable according to the literature in concentrated sulfuric acid, spectroelectrochemical measurements reported in this study provide convincing evidence for its facile conversion into the green-coloured, formally closed shell and, hence, EPR-silent π-bound dimer dication stable in THF at 253 K. The unexpected formation of the thermally unstable dimeric product featuring a characteristic intense low-energy absorption band at 673 nm (1.84 eV; logε_{max}=4.0) is substantiated by ab initio calculations on the parent pleiadiene molecule and the PF_{6}^{-} salts of the corresponding radical cation and dimer dication. The latter is stabilized with respect to the radical cation by 14.40 kcal/mol (DFT B3LYP) [37.64 kcal/mol (CASPT2/DFT B3LYP)]. An excellent match has been obtained between the experimental and TDDFT-calculated UV-vis spectra of the PF_{6}^{-} salt of the pleiadiene dimer dication, considering solvent (THF) effects.

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Department of Chemical Sciences, University of Trieste, Trieste, Italy

Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali, INSTM, Unita di Trieste, Trieste, Italy

The geometrical and electronic structures of a series of small CdSe quantum dots protected by various ligands have been studied by density functional theory. UV-vis spectra have been calculated by time-dependent density functional theory (TDDFT). The goal of this investigation is the rationalization of the basic properties of these systems, in particular, the nature of the exciton peaks. This study has been focused on the (CdSe)x, x = 13, 19, 33, and 66, "magic-size" clusters that are characterized by high stability and large optical gaps. The geometries of the cluster are relaxed both in vacuum and in the presence of the surfactant ligands. To describe the interaction between the bare clusters and the surfactants, model types of ligands are introduced: fatty acids are modeled using formic and acetic acid and amines are modeled using ammonia and methyl amine. Present calculations demonstrate that the ligands play a crucial role in stabilizing the structure in a bulklike geometry and strongly affect the optical gap of the clusters, due to an optimal coordination of the surface atoms. For these "magic-size" clusters, the UV-vis spectrum is calculated at the TDDFT level. The calculated spectra are in good agreement with the experimental ones for clusters with the same dimension capped with the same type of ligands. This suggests that our structures are realistic models of the actual quantum dots.

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Direct evaluation of the induced π current density in [5]paracyclophane (1) shows that, despite the significant non-planarity (*α* = 23.2°) enforced by the pentamethylene bridge, there is only a modest (*ca.* 17%) reduction in the π ring current, justifying the use of shielding-cone arguments for the assignment of ^{1}H NMR chemical shifts of 1 and the claim that the non-planar benzene ring in 1 retains its aromaticity (on the magnetic criterion).

In this tutorial review, we discuss the utilization of chemical shift information as well as ab initio calculations of nuclear shieldings for protein structure determination. Both the empirical and computational aspects of the chemical shift are reviewed and the role of molecular dynamics and the accuracy of different computational methods are discussed. It is anticipated that incorporating theoretical information on chemical shifts will increase the accuracy of protein structures, in the solid and liquid state alike, and extend the applicability of NMR spectroscopy to ever larger systems.

Optical Condensed Matter Physics, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

We performed ab initio quantum chemical calculations for the geometrical and electronic structure of the EDO-TTF (ethylenedioxy-tetrathiafulvalene) molecule using HF, CASSCF and DFT methods. We compare these in vacuo results with the properties of the (EDO-TTF)_{2}PF_{6} crystal at near room temperature. We demonstrate that, by bending and charging the molecule in vacuum, the deformation that is thought to be the origin of charge ordering in this material is an inherent property of the EDO-TTF molecule. We further show that deformations can be readily made at ambient temperatures.

Core electron spectroscopies like X-ray photo-electron spectroscopy, X-ray absorption spectroscopy and electron energy loss spectroscopy are powerful tools to investigate the electronic structure of transition metal, lanthanide and rare earth materials. On the other hand, the interpretation of the spectra is often not straightforward. Relativistic effects and in particular spin-orbit interactions, electron-electron interaction in the valence shell and between core and valence electrons, solid state effects may all play a role in the core electron spectra. Dynamical and non-dynamical electron correlation effects may also be non-negligible. The spectra can be interpreted and predicted using first principles computational methods that take into account both relativity and electron correlation. Furthermore, such approaches enable the interpretation of the complex processes in terms of physical mechanisms. This chapter discusses the effects of relativity on the core spectra of transition metal, lanthanide and actinide materials and a number much used computational approaches to describe and interpret the spectra.

Institucio Catalana de Recerca i Estudis Avancats (ICREA), 08010 Barcelona, Spain

We report here the results for an ab initio approach to obtain the parameters needed for molecular simulations using a polarizable force field. These parameters consist of the atomic charges, polarizabilities, and radii. The former two are readily obtained using methods reported previously (van Duijnen and Swart, J Phys Chem A 1998, 102, 2399; Swart et al. J Comput Chem 2001, 22, 79), whereas here we report a new approach for obtaining atomic second-order radii (SOR), which is based on second-order atomic moments in scaled Voronoi cells. These parameters are obtained from quantumchemistry calculations on the monomers, and used without further adaptation directly for intermolecular interactions. The approach works very well as shown here for four dimers, where high-level coupled cluster with singles and doubles, and perturbative triples (CCSD(T)) and density functional theory (DFT) Swart-Sola`-Bickelhaupt functional including Grimmes dispersion correction (SSB-D) reference data are available for comparison. The energy surfaces for the three methods are very similar, which is also the case for the interaction between a water molecule with either a chloride anion or a sodium cation. These latter systems had previously been used to criticize Tholes damped point-dipole method, but here we show that with the correct use of the method, it is perfectly able to describe the intermolecular interactions. This is most obvious for the induced dipole moment as function of the chloride-oxygen distance, where the direct (discrete) reaction field results are virtually indistinguishable from those obtained at CCSD(T)/aug-cc-pVTZ.

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Departament de Quimica Fisica i Inorganica, Universitat Rovira i Virgili, Marcel-lí Domingo s/n, 43007 Tarragona, Spain

Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluis Companys 23, 08010 Barcelona, Spain

The antisymmetric magnetic interaction is studied using correlated wave function based calculations in oxo-bridged copper bimetallic complexes . All the anisotropic multispin Hamiltonian parameters are extracted using spin-orbit state interaction and effective Hamiltonian theory. It is shown that the methodology is accurate enough to calculate the antisymmetric terms, while the small symmetric anisotropic interactions require more sophisticated calculations. The origin of the antisymmetric anisotropy is analyzed and the effect of geometrical deformations is addressed.

Light-driven molecular rotary motors derived from chiral overcrowded alkenes represent a broad class of compounds for which photochemical rearrangements lead to large scale motion of one part of the molecule with respect to another. It is this motion/change in molecular shape that is employed in many of their applications. A key group in this class are the molecular rotary motors that undergo unidirectional light-driven rotation about a double bond through a series of photochemical and thermal steps. In the present contribution we report a combined quantum chemical and molecular dynamics study of the mechanism of the rotational cycle of the fluorene-based molecular rotary motor 9-(2,4,7-trimethyl-2,3-dihydro-1H-inden-1-ylidene)-9H-fluorene (1). The potential energy surfaces of the ground and excited singlet states of 1 were calculated, and it was found that conical intersections play a central role in the mechanism of photo conversion between the stable conformer of 1 and its metastable conformer. Molecular dynamics simulations indicate that the average lifetime of the fluorene motor in the excited state is 1.40 0.10 ps when starting from the stable conformer, which increases to 1.77 0.13 ps for the reverse photoisomerization. These simulations indicate that the quantum yield of photoisomerization of the stable conformer is 0.92, whereas it is only 0.40 for the reverse photoisomerization. For the first time, a theoretical understanding of the experimentally observed photostationary state of 1 is reported that provides a detailed picture of the photoisomerization dynamics in overcrowded alkene-based molecular motor 1. The analysis of the electronic structure of the fluorene molecular motor holds considerable implications for the design of molecular motors. Importantly, the role of pyramidalization and conical intersections offer new insight into the factors that dominate the photostationary state achieved in these systems.

Institucio Catalana de Recerca i Estudis Avancats (ICREA), 08010 Barcelona, Spain

Applying the classical discrete reaction field (DRF) approach - which includes a treatment for the solution of the many-body polarization in complex systems - the mean atomic polarizability for a Si atom was calculated from the known molecular polarizability of Si_{3}. With only this parameter (6.16 Å^{3}, i.e. close to the free atom value), and the geometries as input, the effective atomic mean polarizabilities and their averages ( <α>_{n}=<α> (n) / n ) for the series Si_{3}-Si_{10} were calculated, and found in excellent agreement with theoretical and experimental values. These <α>_{n} are larger than the bulk value of 3.7Å^{3}.

We used the same input parameter for (by hand) constructed model systems up to n = 4950 with various geometries. For the larger clusters with the diamond lattice, we obtained the bulk value, implying that we 'predicted' the dielectric constant of silicon almost from first principles. However, even the largest system is still too small for considering it as a real dielectric. In other lattices (primitive and face centered cubic), the <α>_{n} are significantly smaller than 3.7Å^{3} which we attribute to the tighter packing in these lattices in comparison with the diamond structure.

The behavior in all these systems can be easily understood by accounting properly for the local fields and for damping the interaction between induced dipoles.

We show that there is no need for additional (e.g. 'charge transfer') parameters.

Using linear response approach to the Mssbauer isomer shift, the calibration constant (57Fe) was obtained from high level ab initio calculations carried out for a representative set of iron compounds. The importance of the effects of relativity and electron correlation for an accurate description of the 57Fe isomer shift is demonstrated on the basis of the HartreeFock, coupled cluster with singles and doubles and of the double hybrid density functional calculations. A reliable value of the calibration constant ( (57Fe) = -0.306 0.009 mm s-1) was obtained with the use of the B2-PLYP double hybrid density functional. This value is in good agreement with the experimentally estimated constant of -0.31 0.04 a0^3 mm s-1 and can be recommended for theoretical modeling of 57Fe isomer shifts.

Equations of the optimized-effective-potential method in a basis set representation are solved with the use of the incomplete Cholesky decomposition technique. The resulting local potential is expanded in terms of the products of occupied and virtual Kohn-Sham orbitals thus avoiding the use of auxiliary basis sets. It is demonstrated that, for a sufficiently large orbital basis set satisfying the condition of linear dependence of these products, stable and numerically accurate solutions of the OEP method can be obtained with the use of the suggested computational approach.

Optical Condensed Matter Physics, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

This study reports in detail the results of systematic large-scale theoretical investigations of the acidic dimeric structural units (D-E, E-F, F-G, and G-H) and pentamer D-E-F-G-H (fondaparinux) of the glycosaminoglycan heparin, and their anionic forms. The geometries and energies of these oligomers have been computed using HF/6-31G(d), Becke3LYP/6-31G(d), and Becke3LYP/ 6-311+G(d,p) methods. The optimized geometries indicate that the most stable structure of these units in the neutral state is stabilized via a system of intramolecular hydrogen bonds. The equilibrium structure of these species changed appreciably upon dissociation. Water has a remarkable effect on the geometry of the anions studied. Because of high negative charge, the solvent effect also resulted in an appreciable energetic stabilization of biologically active anionic forms of these glycosaminoglycans. The stable energy conformations around glycosidic bonds found for dimers and pentamer investigated are compared and discussed with the available experimental X-ray structural data for the structurally related heparinderived pentasaccharides in cocrystals with proteins.

We discuss Ab Initio approaches to calculate the energy lowering (stabilisation) due to aromaticity. We compare the valence bond method and the block-localised wave function approaches to calculate the resonance energy. We conclude that the valence bond approach employs a Pauling-Wheland resonance energy and that the block-localised approach employs a delocalisation criterion. The latter is shown to be more basis set dependent in a series of illustrative calculations

Mssbauer spectroscopy is a widely used analytic tool which provides information about local electronic structure of solid materials on an atomic scale. The isomer shift of resonance nuclear transition is a sensitive parameter which depends on the charge and spin state of the resonating atom as well as on its chemical environment. Theory underlying the isomer shift is reviewed and its connection to the local electronic structure is discussed. A review of advances made in the ab initio calculation of isomer shift is presented. The importance of careful calibration of the parameters of nuclear transitions on the basis of high-level quantum chemical calculations with the inclusion of both relativistic effects and electron correlation is underlined. With the help of accurate theoretical calculations of the isomer shift over a wide range of chemical environments deeper understanding of a relationship between the observed spectroscopic parameters and the electronic structure of materials will be gained.

Nine AuX molecules (X = H, 0, S, Se, Te, F, Cl, Br, I), their isoelectronic HgX+ analogues, and the corresponding neutral HgX diatomics have been investigated using NESC (Normalized Elimination of the Small Component) and B3LYP theory to determine relativistic effects for bond dissociation energies (BDEs), bond lengths, dipole moments, and charge distributions. Relativistic effects are substantially larger for AuX than HgX molecules. AuX bonding has been contrasted with HgX bonding considering the effects of relativity, charge transfer and ionic bonding, 3-electron versus 2-electron bonding, residual -bonding, lone pair repulsion, and the d-block effect. The interplay of the various electronic effects leads to strongly differing trends in calculated BDEs, which can be rationalized with a simple MO model based on electronegativity differences, atomic orbital energies, and their change due to scalar relativity. A relativistic increase or decrease in the BDE is directly related to relativistic changes in the 6s orbital energy and electron density.

Promotores: prof. dr. R. van Leeuwen en prof. dr. R. Broer, 2009

(See also: http://dissertations.ub.rug.nl/faculties/science/2009/a.stan/ or http://irs.ub.rug.nl/ppn/31860969X)

The subject of this thesis lies in the field of many-body theory. This field emerged from the aim to understand the behavior and characterize the properties of many-body systems. When the systems considered are large, the interactions between the elementary constituents of these systems can construct phenomena which may be very different from the behavior of the constituents considered as separated. In an attempt to describe these large systems, these very interactions complicate the description far beyond the computational possibilities. In order to study the collective behavior of the interacting elementary constituents, the complexity of the interaction between them calls for simplifications. All physical approximations made in order to advance in understanding the behavior of many-body systems constitute the field of many-body physics. Within the field of many-body physics, the Green Function Theory describes the behavior and the properties of a system with the aid of an object called the Green function. The Green function is the probability amplitude of finding a particle that has been inserted in the system at (r', t') and removed at (r, t). Since between addition and removal the particle propagated through the system interacting with all other particles, the Green function contains information about its properties. In the Green Function Theory, the interactions of an electronic system i.e. the effects of exchange and correlation, are incorporated into the so called self-energy operator. There are different possible approximations of the self-energy and they completely determine the properties of the system. One of the most widely used approximations of the self-energy is the GW approximation. In this approximation, the self-energy operator is the product of the Green function that describes the propagation of particles and holes in the system, and the dynamically screened interaction which describes how the bare interaction between electrons is modified due to the presence of the other electrons.

The isomer shift for the 23.87 keV M1 resonant transition in the 119Sn nucleus is calibrated with the help of ab initio calculations. The calibration constant (119Sn) obtained from HartreeFock (HF) calculations ( HF(119Sn)=(0.0810.002)a0^ 3 mm/s) and from second-order MllerPlesset (MP2) calculations ( MP2(119Sn)=(0.0910.002)a0^ 3 mm/s) are in good agreement with the previously obtained values. The importance of a proper treatment of electron correlation effects is demonstrated on the basis of a statistical analysis of the results of the calibration. The approach used in the calibration is applied to study the 119Sn isomer shift in CaSnO3 perovskite under pressure. Comparison with the experimental results for the pressure range of 036 GPa shows that the current methodology is capable of describing tiny variations of isomer shift with reasonable accuracy.

European Theoretical Spectroscopy Facility (ETSF)

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

We perform GW calculations on atoms and diatomic molecules at different levels of selfconsistency and investigate the effects of self-consistency on total energies, ionization potentials and on particle number conservation. We further propose a partially self-consistent GW scheme in which we keep the correlation part of the self-energy fixed within the self-consistency cycle. This approximation is compared to the fully self-consistent GW results and to the GW_{0} and the G_{0}W_{0} approximations. Total energies, ionization potentials and two-electron removal energies obtained with our partially self-consistent GW approximation are in excellent agreement with fully selfconsistent GW results while requiring only a fraction of the computational effort. We also find that self-consistent and partially self-consistent schemes provide ionization energies of similar quality as the G_{0}W_{0} values but yield better total energies and energy differences.

European Theoretical Spectroscopy Facility (ETSF)

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

We have developed a time propagation scheme for the Kadanoff-Baym equations for general inhomogeneous systems. These equations describe the time evolution of the nonequilibrium Green function for interacting many-body systems in the presence of time-dependent external fields. The external fields are treated nonperturbatively whereas the many-body interactions are incorporated perturbatively using Φ-derivable self-energy approximations that guarantee the satisfaction of the macroscopic conservation laws of the system. These approximations are discussed in detail for the time-dependent Hartree-Fock, the second Born and the GW approximation.

Potential energy surfaces of the ground and the first excited singlet states of the (3R,3 R)-(P,P)-trans-1,1 ,2,2 ,3,3 ,4,4 -octahydro-3,3 -dimethyl-4,4 -biphenanthrylidene rotary molecular motor have been investigated along the central C4=C4 double bond twisting mode starting from the (P,P)-trans and from the (P,P)-cis conformations occurring in the photoisomerization cycle of this compound. The potential energy profiles obtained with the help of the state average spin restricted ensemble-referenced Kohn Sham (SA-REKS) method feature minima on the excited state surface, the positions of which are displaced with respect to the barriers on the ground state surface toward the isomerization products, the (M,M)-cis and the (M,M)-trans conformations, respectively. The origin of these minima is analyzed and explained. The results of the present study suggest that the experimentally observed unidirectionality of photoinduced rotation in the above compound can be corroborated by the obtained profiles of the ground and excited state potential energy surfaces.

Department of Chemistry, University of Sheffield, Sheffield, S3 7HF, UK

Ring-current maps give an immediate visualisation of aromaticity on the magnetic criterion-by which a cyclic system that supports diatropic (paratropic) current induced by a perpendicular magnetic field is aromatic (anti-aromatic). Calculations of maps with the ipsocentric choice of origin are made in the 6-31G** basis set at Hartree-Fock (HF) and density functional (DFT) levels (PW91 and B3LYP functionals) on porphyrin, porphycene, orangarin, sapphyrin and hexabenzocoronene. In these systems, DFT and HF approaches produce optimal geometries with different point-group symmetries and/or different patterns of bond alternation. The ring-current maps derived with all four combinations of methods indicate that the main features of the current (global nature, direction, estimated strength) survive in systems with symmetry-breaking, but that choice of geometry is more critical for the detail of the current than is the electronic-structure method.

Thin films of TbMnO3 have been grown on SrTiO3 substrates. The films grow under compressive strain and are only partially clamped to the substrate. This produces remarkable changes in the magnetic properties and, unlike the bulk material, the films display ferromagnetic interactions below the ordering temperature of ~40 K. X-ray photoemission measurements in the films show that the Mn 3s splitting is 0.3 eV larger than that of the bulk. Ab initio embedded-cluster calculations yield Mn 3s splittings that are in agreement with the experiment and reveal that the larger observed values are due to a larger ionicity of the films.

The effect of electron-electron scattering on the equilibrium properties of few-electron quantum dots is investigated by means of nonequilibrium Green's function theory. The ground and equilibrium states are self-consistently computed from the Matsubara imaginary time Green's function for the spatially inhomoge- neous quantum dot system whose constituent charge carriers are treated as spin-polarized. To include correlations, the Dyson equation is solved, starting from a Hartree-Fock reference state, within a conserving second-order self-energy approximation where direct and exchange contributions to the electron-electron interaction are included on the same footing. We present results for the zero and finite temperature charge carrier densities, the orbital-resolved distribution functions, and the self-consistent total energies and spectral functions for isotropic two-dimensional parabolic confinement as well as for the limit of large anisotropyquasi-one- dimensional entrapment. For the considered quantum dots with N=2, 3, and 6 electrons, the analysis comprises the crossover from Fermi gas or liquid at large carrier density to Wigner molecule or crystal behavior in the low-density limit.

ICREA, Department of Physical and Inorganic Chemistry, Universitat Rovira i Virgili, Marcel-li Domingo s/n, 43007 Tarragona, Spain

Multiconfigurational wave functions are calculated for a series of Fe complexes. We find a linear correlation between the experimental ^{57}Fe Mossbauer isomer shift and the calculated electron density at the Fe nucleus. However, the analysis of the wave function in valence bond terms shows that there is no straightforward relation between the density at the nucleus and the Fe charge. The analysis of the CASSCF wave function expressed in localized orbitals shows that the isomer shift is very sensitive to the weight of charge transfer con- figurations and hence to the covalency, rather than to the absolute charge.

Bond dissociation energies (BDEs) of neutral HgX and cationic HgX+ molecules range from less than a kcal mol-1 to as much as 60 kcal mol-1. Using NESC/CCSD(T) [normalized elimination of the small component and coupled-cluster theory with all single and double excitations and a perturbative treatment of the triple excitations] in combination with triple-zeta basis sets, bonding in 28 mercury molecules HgX (X=H, Li, Na, K, Rb, CH3, SiH3, GeH3, SnH3, NH2, PH2, AsH2, SbH2, OH, SH, SeH, TeH, O, S, Se, Te, F, Cl, Br, I, CN, CF3, OCF3) and their corresponding 28 cations is investigated. Mercury undergoes weak covalent bonding with its partner X in most cases (exceptions: X=alkali atoms, which lead to van der Waals bonding) although the BDEs are mostly smaller than 12 kcal mol-1. Bonding is weakened by 1) a singly occupied destabilized *-HOMO and 2) lone pair repulsion. The magnitude of *-destabilization can be determined from the energy difference BDE(HgX)-BDE(HgX+), which is largest for bonding partners from groups IVb and Vb of the periodic table (up to 80 kcal mol-1). BDEs can be enlarged by charge transfer from Hg and increased HgX ionic bonding, provided the bonding partner of Hg is sufficiently electronegative. The fine-tuning of covalent and ionic bonding, -destabilization, and lone-pair repulsion occurs via relativistic effects where 6s AO contraction and 5d AO expansion are decisive. Lone pair repulsion involving the mercury 5d AOs plays an important role in the case of some mercury chalcogenides HgE (E=O, Te) where it leads to 3 rather than 1 + ground states. However, both HgE(3 ) and HgE(1 +) should not be experimentally detectable under normal conditions, which is in contrast to experimental predictions suggesting BDE values for HgE between 30 and 53 kcal mol-1. The results of this work are discussed with regard to their relevance for mercury bonding in general, the chemistry of mercury, and reactions of elemental Hg in the atmosphere.

Promotor: prof. dr. R. Broer Copromotores: dr. ir. P. L. de Boeij dr. R. van Leeuwen, 2008

In this thesis we developed the time-dependent version of the multicomponent density functional approach to treat time-dependent electron-nuclear systems. The method enables to describe the electron-nuclear coupling fully quantum mechanically. No Born-Oppenheimer approximation is involved in the approach. The multicomponent density functional theory is formulated for an electron-nuclear system in the body-fixed coordinate frame attached to the nuclei. It allows us to describe properly the internal properties of the system. The nuclei in the system are described by the diagonal of many-body density matrix which depends on all nuclear coordinates. In the Kohn-Sham picture this density matrix is calculated from an equation with a time-dependent potential that depends on all nuclear coordinates. For the diatomic molecule in the stationary case this potential turns out to be very close to the familiar Born-Oppenheimer potential. However, the Kohn-Sham scheme goes much beyond the Born-Oppenheimer picture in allowing an exact quantum description of the motion of the nuclei. As a consequence of the body-fixed frame transformation the external potential acting on the electrons, which is a one-body potential in the laboratory frame, becomes a many-body potential u with respect to the nuclear coordinates in the body-fixed frame.

Promotores: prof. dr. R. Broer and prof. C. de Graaf, 2008

This dissertation presents the results of theoretical investigations of the electron distribution in crystals. Electronic and magnetic properties can be investigated using the theory of quantum mechanics developed at the beginning of the 20th century. The increased power of computational tools enables nowadays the treatment of more and more accurate models to simulate electron motion in crystals or molecules and complement experimental observations and interpretations. Crystals containing transition metal elements are studied in many laboratories for the complex behavior of electrons leading in some cases to intriguing properties like superconductivity or magnetoresistance. These properties are often intimately connected to the open shell character of the transition metal ions, which are susceptible to adopt different electronic configurations and oxidation states depending on their environment. This dissertation focuses on the distribution of electrons between transition metal and ligands and discusses various definitions of oxidation state and charges for the transition metal ions and compares with estimates on the basis of different experimental techniques.

Institucio Catalana de Recerca i Estudis Avancats (ICREA), 08010 Barcelona, Spain and Institut de Qumica Computacional, Universitat de Girona, Campus Montilivi, 17071 Girona, Spain

Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA

We present here the discrete reaction field (DRF) approach, which is an accurate and efficient model for studying solvent effects on spectra, chemical reactions, solute properties, etc. The DRF approach uses a polarizable force field, which is (apart from the short-range repulsion) based entirely on second-order perturbation theory, and therefore ensures the correct analytical form of model potentials. The individual interaction components are modeled independently from each other, in a rigorous and straightforward way. The required force field parameters result as much as possible from quantum-chemical calculations and on monomer properties, thereby avoiding undesired fitting of these parameters to empirical data. Because the physical description is correct and consistent, the method allows for arbitrary division of a system into different subsystems, which may be described either on the quantum-mechanical (QM) or the molecular mechanics (MM) level, without significant loss of accuracy. This allows for performing fully MM molecular simulations (Monte Carlo, molecular dynamics), which can subsequently be followed by performing QM/MM calculations on a selected number of representative snapshots from these simulations. These QM/MM calculations then give directly the solvent effects on emission or absorption spectra, molecular properties, organic reactions, etc.

Department of Chemical Physics and Materials Science Centre, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Pacific Northwest National Laboratory, 902 Battelle Boulevard, P.O. Box 999, Richland, WA 99352, USA

In a previous study of the 3s X-ray photoelectron spectra, XPS, of Mn, we identified a new intraatomic many-body effect that lead to an ~50% increase in the predicted exchange splitting of the main high spin and low spin XPS peaks. The new many-body effect involved the promotion of one electron from the M shell, 3s, 3p, and 3d, into a 4f orbital and a redistribution of the remaining electrons over the M shell orbitals; of particular importance were frustrated Auger configurations. FAC's where the 3s shell was filled. In the present work, we demonstrate the general importance of these 4f FAC's by showing that they are of comparable importance for increasing the 3s exchange splitting in Ni as they were in Mn.

Calculations on crystalline organic radicals were performed to establish the ground states of these materials. These calculations show that the radicals may interact, depending on their orientation in the crystal structure. For galvinxoyl, a second structure is proposed which is similar to that of azagalvinoxyl, in which the radicals form pairs. This structure accounts for the anomalous magnetic properties of galvinoxyl at low temperatures.

A periodic density functional theory method using the B3LYP hybrid exchange-correlation potential is applied to the Prussian blue analogue RbMn[Fe(CN)_{6}] to evaluate the suitability of the method for studying, and predicting, the photomagnetic behavior of Prussian blue analogues and related materials. The method allows correct description of the equilibrium structures of the different electronic configurations with regard to the cell parameters and bond distances. In agreement with the experimental data, the calculations have shown that the low-temperature phase is the stable phase at low temperature instead of the high-temperature phase. Additionally, the method gives an estimation for the enthalpy difference (HT - LT) with a value of 143 J mol^{-1} K^{-1}. The comparison of our calculations with experimental data from the literature and from our calorimetric and X-ray photoelectron spectroscopy measurements on the Rb_{0.97}Mn[Fe(CN)_{6}]_{0.98} 1.03H_{2}O compound is analyzed, and in general, a satisfactory agreement is obtained. The method also predicts the metastable nature of the electronic configuration of the high-temperature phase, a necessary condition to photoinduce that phase at low temperatures. It gives a photoactivation energy of 2.36 eV, which is in agreement with photoinduced demagnetization produced by a green laser.

The orbital products of occupied and virtual orbitals are employed as an expansion basis for the charge density generating the local potential in the optimized effective potential method thus avoiding the use of auxiliary basis sets. The high computational cost arising from the quadratic increase of the dimension of this product basis with system size can be greatly reduced by elimination of the linearly dependent products according to a procedure suggested earlier. Numerical results from this approach show a very good agreement with those obtained from balancing the auxiliary basis for the expansion of the local potential with the orbital basis set.

With the help of a recently suggested computational scheme, Mssbauer isomer shifts are calculated within the context of density functional theory, for a series of iron containing compounds. The influence of the choice of a density functional and of the truncation of a basis set on the results of calculations is analyzed. It has been observed that the hybrid density functionals, especially BH&HLYP, provide better correlation with experimental results than pure density functionals. The analysis of basis set truncation reveals that the addition (or removal) of the tightmost primitive functions to a large uncontracted basis set has only a minor influence on the calculated isomer shift values. It is observed that, with the use of a small contracted basis set, a reasonable accuracy for the calculated isomer shifts can be achieved.

A time-independent density functional approach to the calculation of excitation energies from the ground states of molecules typified by the strong nondynamic electron correlation is suggested. The new method is based on the use of the spin-restricted ensemble-referenced Kohn-Sham formalism for the calculation of the ground state. In the new method, the average energy of the ground state and a state created by a single excitation thereof is minimized with respect to the Kohn-Sham orbitals and their fractional occupation numbers. The lowest singlet excitation energies obtained with the help of the new formalism for a number of model systems, such as the hydrogen molecule with stretched bond, twisted ethylene, and twisted hexa-1,3,5-triene, are compared with the results of the time-dependent density functional theory, with the results of ab initio CASSCF/CASPT2 calculations, and with the experimental data. Applicability of the new method to the description of photochemical reactions is discussed.

High-level ab initio calculations at the coupled cluster with single and double substitutions and perturbative treatment of triple substitutions, CCSD(T), level of theory have been carried out for the dimers of coinage metal atoms Cu, Ag, and Au in the ground 1Sigma(g)+ state and in the excited 3Sigma(u)+ state. All of the calculations have been carried out with the inclusion of scalar-relativistic effects via the normalized elimination of the small component (NESC) method. For the dimers in the triplet state, nonzero bond dissociation energies are obtained which vary from 1.3 kcal/mol for 3Cu2 to 4.6 kcal/mol for 3Au2. Taking into account that, in bulky high-spin copper clusters, the bond dissociation energy per atom increases steeply to the value of ca. 19 kcal/mol, the results obtained in the present paper suggest that the bond dissociation energy per atom in high-spin gold clusters may reach extremely high values exceeding 20 kcal/mol thus becoming comparable to the usual bonding due to the spin-pairing mechanism.

Zenike Institute for Advanced Materials, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Novel mononuclear, trinuclear, and hetero-trinuclear supermolecular complexes, [Co(phen)_{2}(H_{2}O)(HTST)].2H_{2}O (1), [Co_{3}(phen)_{6}(H_{2}O)_{2}(TST)_{2}].7H_{2}O (2), and [Co_{2}Cu(phen)_{6}(H_{2}O)_{2}(TST)_{2}].10H_{2}O (3), have been synthesized by the reactions of a new tri-sulfonate ligand (2,4,6-tris(4-sulfophenylamino)-1,3,5-triazine, H_{3}TST) with the M^{2+} (M=Co, Cu) and the second ligand 1,10-phenanthroline (phen). Complex 1 contains a cis-Co(II)(phen)_{2} building block and an HTST as monodentate ligand; complex 2 consists of two TST as bidentate ligands connecting one trans- and two cis-Co(II)(phen)2 building blocks; complex 3 is formed by replacing the trans-Co(II)(phen)_{2} in 2 with a trans-Cu(II)(phen)_{2}, which is the first reported hetero-trinuclear supramolecular complex containing both the Co(II)(phen)_{2} and Cu(II)(phen)_{2} as building blocks. The study shows the flexible multifunctional self-assembly capability of the H_{3}TST ligands presenting in these supramolecular complexes through coordinative, H-bonding and even *π*–*π* stacking interactions. The photoluminescent optical properties of these complexes are also investigated and discussed as well as the second-order nonlinear optical properties of 1

In this report, we will give an overview of Density Matrix Functional Theory (DMFT). In the first part we will discuss the extended Hohenberg-Kohn theorem for non-local potentials, which claims a one-one mapping between the groundstate wavefunction and the reduced density one-matrix. The eigenequations for the natural orbitals are derived, which provide a means to apply DMFT in practice. The main part of the report consists of the discussion of several functionals which have been proposed to include electron correlation. Of the 5 discussed functionals (BB, GU, BBC1, BBC2 and BBC3), BBC3 performs best for the potential energy curves of small molecules. It is however not applicable to infinite systems, such as the homogeneous electron gas. A universal functional remains to be found.

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The hydrothermal reaction of Co(NO_{3})_{2}.6H_{2}O and a newly designed ligand H_{2}DCNT yields a three-dimensional polymer [Co(DCNT)(H_{2}O)]_{n} (1), H_{2}DCNT=2,4-bis(4-carboxyphenylamino)-6-diethylamino-1,3,5-triazine. In the structure of 1, each DCNT_{2-} has three coordination sites, one nitrogen atom in the triazine ring coordinating to Co(II) and two carboxylates adopting _{_2}-bridging mode, which make the infinite Co(II) chains array uniformly and evenly towards the crystallographic *c* axis. Luminescent and magnetic properties of 1 were also studied.

Zenike Institute for Advanced Materials, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The novel supramolecular silver(I) compound with formula [Ag_{6}(TST)_{2}(bipy)_{6}(H_{2}O)_{2}]_{n} . 3*n*H_{2}O (**1**) based on assembly of Ag(I) and mixed ligand bipy/TST^{3−}, bipy = 2,2′-bipyridine, H_{3}TST = 2,4,6-tris(4-sulfophenylamino)-1,3,5-triazine, has been prepared by hydrothermal method. In the solid-state structure of **1**, two-dimensional layered polymeric structures extended with subunits [Ag_{6}(TST)_{2}(bipy)_{6}(H_{2}O)_{2}] interact each other in the form of π–π attractions between bipy, forming a three-dimensional supramolecular architecture. Compound **1** represents a Ag-containing polymeric compound possessing room-temperature luminescence.

ICREA, Department of Physical and Inorganic Chemistry, Universitat Rovira i Virgili, Marcel-li Domingo s/n, 43007 Tarragona, Spain

A simple scheme is proposed to analyze the *N*-electron wave function obtained in embedded cluster calculations in valence bond terms such as ligand-to-metal charge transfer and non charge transfer determinants. The analysis is based on a unitary transformation of pairs of natural orbitals to optimal atomic-like orbitals. The procedure is applied to compare the degree of ionicity in NiO and MnO, and to explain the existence or absence of Jahn-Teller distortions in LaMnO_{3}, CaMnO_{3} and CaFeO_{3}. We find that the ground state of LaMnO_{3} is dominated by non charge transfer configurations, whereas the charge transfer configurations dominate the ground state wave function in the other two perovskites.

Quantum Theory Project, University of Florida, Gainesville, Florida 32611-7200

We report the calculated visible spectrum of [Fe^{III}(PyPepS)_{2}]^{-} in aqueous solution. From all-classical molecular dynamics simulations on the solute and 200 water molecules with a polarizable force field, 25 solute/solvent configurations were chosen at random from a 50 ps production run and subjected the systems to calculations using time-dependent density functional theory TD-DFT for the solute, combined with a solvation model in which the water molecules carry charges and polarizabilities. In each calculation the first 60 excited states were collected in order to span the experimental spectrum. Since the solute has a doublet ground state several excitations to states are of type three electrons in three orbitals, each of which gives rise to a manifold of a quartet and two doublet states which cannot properly be represented by single Slater determinants. We applied a tentative scheme to analyze this type of spin contamination in terms of Δ and Δ transitions between the same orbital pairs. Assuming the associated states as pure single determinants obtained from restricted calculations, we construct conformation state functions CFSs , i.e., eigenfunctions of the Hamiltonian S_{z} and S_{2}, for the two doublets and the quartet for each Δ,Δ pair, the necessary parameters coming from regular and spin-flip calculations. It appears that the lower final states remain where they were originally calculated, while the higher states move up by some tenths of an eV. In this case filtering out these higher states gives a spectrum that compares very well with experiment, but nevertheless we suggest investigating a possible re formulation of TD-DFT in terms of CFSs rather than determinants.

In this paper we derive the relativistic two-component formulation of time-dependent current-density-functional theory. To arrive at a two-component current-density formulation we apply a Foldy-Wouthuysen-type transformation to the time-dependent four-component Dirac-Kohn-Sham equations of relativistic density-functional theory. The two-component Hamiltonian is obtained as a regular expansion which is gauge invariant at each order of approximation, and to zeroth order it represents the time-dependent version of the relativistic zeroth order regular Hamiltonian obtained by van Lenthe et al., for the ground state J. Chem. Phys. 99, 4597 1993 . The corresponding zeroth order regular expression for the density is unchanged, whereas the current-density operator now comprises a paramagnetic, a diamagnetic, and a spin contribution, similar to the Gordon decomposition of the Dirac four current. The zeroth order current density is directly related to the mean velocity corresponding to the zeroth order Hamiltonian. These density and current density operators satisfy the continuity equation. This zeroth order approximation is therefore consistent and physically realistic. By combining this formalism with the formulation of the linear response of solids within time-dependent current-density functional theory Romaniello and de Boeij, Phys. Rev. B 71, 155108 2005 , we derive a method that can treat orbital and spin contributions to the response in a unified way. The effect of spin-orbit coupling can now be taken into account. As first test we apply the method to calculate the relativistic effects in the linear response of several metals and nonmetals to a macroscopic electric field. Treatment of spin-orbit coupling yields visible changes in the spectra: a smooth onset of the interband transitions in the absorption spectrum of Au, a sharp onset with peak at about 0.46 eV in the absorption spectrum of W, and a low-frequency doublet structure in the absorption spectra of ZnTe, CdTe, and HgTe in agreement with experimental results.

The optimized effective potential (OEP) equations are solved in a matrix representation using the orbital products of occupied and virtual orbitals for the representation of both the local potential and the response function. This results in a direct relationship between the matrix elements of local and nonlocal operators for the exchange-correlation potential. The effect of the truncation of the number of such products in the case of finite orbital basis sets on the OEP orbital and total energies and on the spectrum of eigenvalues of the response function is examined. Test calculations for Ar and Ne show that rather large AO basis sets are needed to obtain an accurate representation of the response function.

A quantum chemical computational scheme for the calculation of isomer shift in Mssbauer spectroscopy is suggested. Within the described scheme, the isomer shift is treated as a derivative of the total electronic energy with respect to the radius of a finite nucleus. The explicit use of a finite nucleus model in the calculations enables one to incorporate straightforwardly the effects of relativity and electron correlation. The results of benchmark calculations carried out for several iron complexes as well as for a number of atoms and atomic ions are presented and compared with the available experimental and theoretical data.

The performance of density functional theory in estimating the magnetic coupling constant in a series of Cu(II) binuclear complexes is investigated by making use of two open shell formalisms: the broken symmetry and the spin-restricted ensemble-referenced Kohn-Sham methods. The strong dependence of the calculated magnetic coupling constants with respect to the exchange-correlation functional is confirmed and found to be independent of whether spin symmetry is imposed or not. The use of a method which guarantees the spin state does not improve the correlation with the experiment and indeed shows some worsening due to an overestimation of the ferromagnetic interactions. However, with the present exchange-correlation functionals, a rather systematic deviation is found. Therefore, it would be possible to develop improved density functionals which will allow for a rigorous treatment of open shell systems in density functional theory.

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

German Cancer Research Center, Central Spectroscopy, B090, Im Neuenheimer Feld 280, D-69120 Heidelberg, Germany

DFT was used to investigate molecular structure and metal affinity of the systems CH_{3}CO_{2}M (1), CH_{3}-O-SO_{3}M (2), CH_{3}-NH-SO_{3}M (3), (CH_{3}-O-PO_{3}M)^{-} (4), CH_{3}-O-PO_{3}M_{2} (5), CH_{3}-O-(CH_{3})PO_{2}M (6), and 1,4-DiOMe IdoA-2SM_{2} (7; ^{2}S_{o} conformation) (M = Li^{+} and Na^{+}), respectively. Interaction enthalpies, entropies and Gibbs energies of the metal-coordinated systems were determined on the B3LYP/6-311+G(d,p) level. The computed Gibbs energies, ΔG^{o}, of the isolated systems 1-7 are negative and span a rather broad energy interval (from 500 to 1500 kJ mol^{-1}). The lithium and sodium binding enthalpies and Gibbs energies of a series of phosphate, carboxylate, N-, and O-sulfate anions indicate that multidentate chelation plays an important role in the binding. In particular, the glycosaminoglycan structural unit of heparin 1,4-DiOMe IdoA-2SM2 (M = Li^{+} and Na^{+}) with coordinating groups in the hexopyranose ring exhibits enhanced metal ion binding energies. Computations that include the effect of solvation showed that in water the relative stability of Li^{+}... Ligand and Na^{+}... Ligand ionic bonds is rapidly diminished. The computed interaction Gibbs energy in water is small, slightly negative and/or positive, i.e. destabilizing. Thus in water, both contact ion pairs and solvent-separated ion pairs may coexist.

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Analytic energy gradients with respect to atomic coordinates for systems with translational invariance are formulated within the framework of Kohn-Sham Density Functional Theory. The energy gradients are implemented in the BAND program for periodic DFT calculations which directly employs Bloch basis set made up of Slater-type (STOs) and numeric atomic orbitals (NAOs). The details of our implementation are described including the use of symmetry in the reciprocal and direct spaces, as well as the application of the frozen core approximation.

In this work, we investigate the Vignale-Kohn current functional when applied to the calculation of optical spectra of semiconductors. We discuss our results for silicon. We found qualitatively similar results for other semiconductors. These results show that there are serious limitations to the general applicability of the Vignale-Kohn functional. We show that the constraints on the degree of nonuniformity of the ground-state density and on the degree of the spatial variation of the external potential under which the Vignale-Kohn functional was derived are almost all violated. We argue that the Vignale-Kohn functional is not suited to use in the calculation of optical spectra of semiconductors since the functional was derived for a weakly inhomogeneous electron gas in the region above the particle-hole continuum, whereas the systems we study are strongly inhomogeneous and the absorption spectrum is closely related to the particle-hole continuum.

ICREA, Department of Physical and Inorganic Chemistry, Universitat Rovira i Virgili, Marcel-li Domingo s/n, 43007 Tarragona, Spain

Ab initio calculations have been performed to clarify the character of the ground state of the high temperature phase of CaFeO_{3} at different external pressures. The analysis of the correlated N-electron wave function of properly embedded FeO_{6} clusters in terms of optimal atomic orbitals clearly establishes the character of the ground state as being dominated by charge transfer configurations. For all pressures, the number of Fe 3*d* electrons is around 5 and iron should be considered as a Fe^{3+} ion. We find a S=2 to S=1 transition around 25 GPa in the CaFeO_{3} crystal.

IFW Dresden, D-01171 Dresden, Germany

Laboratory of Crystallography, University of Bayreuth, 95440 Bayreuth, Germany

Institut für Anorganische Chemie, Universität zu Köln, 50937 Köln, Germany

The sequence of phase transitions and the symmetry of in particular the low temperature incommensurate and spin-Peierls phases of the quasi one-dimensional inorganic spin-Peierls system TiOX (X=Br and Cl) have been studied using inelastic light scattering experiments. The anomalous first-order character of the transition to the spin-Peierls phase is found to be a consequence of the different symmetries of the incommensurate and spin-Peierls (P2_{1}/*m*) phases. The pressure dependence of the lowest transition temperature strongly suggests that magnetic interchain interactions play an important role in the formation of the spin-Peierls and the incommensurate phases. Finally, a comparison of Raman data on VOCl to the TiOX spectra shows that the high energy scattering observed previously has a phononic origin.

University of Groningen, Theoretical Chemistry, Zernike Institute for Advanced Materials, 9747AG, Nijenborgh 4, Groningen, The Netherlands

Institut für Theoretische Physik II, Universität Erlangen-Nrnberg, Staudtstrasse 7, D-91058 Erlangen, Germany

We demonstrate that the time-dependent Krieger-Li-Iafrate approximation in combination with the exchange-only functional violates the zero-force theorem. By analyzing the time-dependent dipole moment of Na_{5} and Na_{9}^{+}, we furthermore show that this can lead to an unphysical self-excitation of the system depending on the system properties and the excitation strength. Analytical aspects, especially the connection between the zero-force theorem and the generalized-translation invariance of the potential, are discussed.

Department of Physics, University of Jyväskylä, FI-40014, Survontie 9, Jyväskylä, Finland

Institut für Theoretische Physik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany

We derive the basic formalism of density functional theory for time-dependent electron-nuclear systems. The basic variables of this theory are the electron density in body-fixed frame coordinates and the diagonal of the nuclear N-body density matrix. The body-fixed frame transformation is carried out in order to achieve an electron density that reflects the internal symmetry of the system. We discuss the implications of this body-fixed frame transformation and establish a Runge-Gross type theorem and derive Kohn-Sham equations for the electrons and nuclei. We illustrate the formalism by performing calculations on a one-dimensional diatomic molecule for which the many-body Schroedinger equation can be solved numerically. These benchmark results are then compared to the solution of the time-dependent Kohn-Sham equations in the Hartree approximation. Furthermore, we analyze the excitation energies obtained from the linear response formalism in the single pole approximation. We find that there is a clear need for improved functionals that go beyond the simple Hartree approximation.

A crystalline sample of TiOBr is probed at room temperature by a combination of electron spectroscopies and the results are compared to theoretical embedded-cluster calculations. Resonant photoemission of the valence band confirms that the lowest binding energy feature arises from the singly occupied Ti 3d orbital. The polarization dependence of this orbital in nonresonant photoemission is consistent with the expected dominant d(y)(2)-z(2) character. The analysis of the Ti L-2,L-3 x-ray absorption spectra confirms the complete splitting of the Ti 3d shell. X-ray absorption and resonant photoemission at the O 1s edge provide direct evidence for hybridization between the transition metal orbitals and the O 2p levels, which leads to superexchange interactions between the Ti ions. The existence of a mixing of O and Ti states and of strong superexchange interactions is supported by calculations of the ground-state electronic and magnetic properties. The calculated superexchange interchain interaction is one fifth in strength of the total magnetic coupling along the chain, and is antiferromagnetic in character. This O-mediated interchain interaction is frustrated in the room temperature phase of TiOBr and thus couples strongly to distortions of the soft lattice. The competition between the interchain magnetoelastic coupling and the spin-Peierls interaction might be at the origin of the complex TiOX phase diagram.

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The center-of-mass (c.m.) oscillation of a many-body system in a harmonic trap is known to be independent of the interparticle interaction. However, this is not necessarily the case if the interactions are treated approximately. Here, we prove a simple general criterion for preservation of the c.m. mode: the approximation has to preserve density and momentum. The result equally applies to zero and finite temperatures, as well as to nonequilibrium situations, and to the linear and nonlinear response regimes.

Application to Atoms and Molecules

We implement time propagation of the nonequilibrium Green function for atoms and molecules by solving the Kadanoff-Baym equations within a conserving self-energy approximation. We here demonstrate the usefulness of time propagation for calculating spectral functions and for describing the correlated electron dynamics in a nonperturbative electric field. We also demonstrate the use of time propagation as a method for calculating charge-neutral excitation energies, equivalent to highly advanced solutions of the Bethe-Salpeter equation.

The convergence behavior of the iterative solution of the normalized elimination of the small component (NESC) method is investigated. A simple and efficient computational protocol for obtaining the exact positive-energy eigenvalues of the relativistic Hamiltonian starting from the energies obtained within the regular approximation is suggested. The protocol is based on the analysis of the relationship between the eigenvalues of the quasi-relativistic Hamiltonian in the regular approximation and the positive-energy eigenvalues of the exact relativistic Hamiltonian which was derived in the course of this work.

Department of Physical and Inorganic Chemistry, Universitat Rovira i Virgili, Marcelli Domingo, Tarragona 43007, Spain

Department of Chemistry, University of North Texas, Denton, Texas 76203-5070, USA

The origin of the features in the Ni 3s X-ray photoelectron spectrum of NiO is investigated using a non-orthogonal configuration interaction approach for an embedded [NiO_{6}] cluster. We study the interplay of inter-atomic screening with the metal core hole and intra-atomic exchange and electron correlation effects. We show that the spectrum can be described in terms of only few key configurations, provided that orbital relaxation effects are explicitly taken into account for the excited charge transfer configurations. The strength of this approach has been demonstrated earlier for those final states that have a high-spin coupling. In the present contribution the analysis is extended to include low-spin coupled 3s-hole states. The effects of enlarging the embedded cluster and of an improved representation of the nearest cluster surroundings were studied for the high-spin final states. We found only minor effects on the computed peak separations.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Department of Physics, Università degli Studi di Trieste, Via Valerio, 2 I-34127, Trieste, Italy and Sincrotrone Trieste, I-34012, Basovizza, Trieste, Italy

This Letter addresses a long-standing problem related to non-dynamical electron correlation effects. The origin of the large differential electronic correlation energy among the neutral 1S ground state, the lowest, 2P, ionic state and the first excited, 2S, ionic state of the Ne and Ar atoms is explained in terms of the near degeneracy of low-lying excited configurations. There is an anomalous correlation for the 2S state that is shown to be due to non-dynamical correlation involving a low-lying excited configuration. The conceptual framework used here is also appropriate to be used for other atomic and molecular systems.

Stratingh Institute of Chemistry and Chemical Engineering, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Department of Physics, University of Calabria, via P. Bucci, 87036, Arcavacata di Rende (Cs) Italy

Department of Physics, University of Ioannina, 45110 Ioannina, Greece

The synthesis and detailed characterization of a few samples of the compound Rb_{x}Mn[Fe(CN)_{6}]_{y}^{.}zH_{2}O are described. The composition of the materials significantly depends on the applied preparative conditions. Analysis of spectroscopic results (FTIR, Raman, ^{57}Fe Mössbauer, XPS) and X-ray powder diffraction data yielded a further assessment of the difference in structural features in terms of the amount of Fe(CN)_{6} vacancies and the associated number of water molecules. The characteristic individual magnetic behavior, as well as the metal-to-metal charge transfer capabilities of the various samples could be related to significant changes within the structures which appear to be associated with the synthetic method used.

No abstract available

No abstract available

No abstract available

We solve the Dyson equation for atoms and diatomic molecules within the *GW* approximation, in order to elucidate the effects of self-consistency on the total energies and ionization potentials. We find *GW* to produce accurate energy differences although the selfconsistent total energies differ significantly from the exact values. Total energies obtained from the Luttinger-Ward functional E_{LW}[G] with simple, approximate Green functions as input, are shown to be in excellent agreement with the self-consistent results. This demonstrates that the Luttinger-Ward functional is a reliable method for testing the merits of different self-energy approximations without the need to solve the Dyson equation self-consistently. Self-consistent *GW* ionization potentials are calculated from the Extended Koopmans Theorem, and shown to be in good agreement with the experimental results. We also find the self-consistent ionization potentials to be often better than the non-self-consistent *G*_{0}*W*_{0} values. We conclude that *GW* calculations should be done self-consistently in order to obtain physically meaningful and unambiguous energy differences.

Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

n.a.

Theoretical Chemistry, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Department of Chemistry, State University ofNewYork at Buffalo, 312 Natural Science Complex, Buffalo, New York 14260-3000, USA

In this work we investigate the circular dichroism (CD) spectrum of [Co(en)3]3+ in water, using the discrete solvent reaction field (DRF) model. The DRF model is a polarizable quantum mechanics/molecular mechanics (QM/MM) model. The implementation of the DRF model for CD spectra calculations based on time-dependent density functional theory (TDDFT) is presented. The combination of DRF with TDDFT allows for a computationally attractive solution for calculating chirooptical properties of molecules in solution when explicit solvent structures are of interest. Using a mixed coarse/fine-grained parallel computation, we show that average CD spectra from snapshots of the solvent structure can be obtained routinely. Classical polarizable molecular dynamics (MD) simulations have been used to obtain the solvent structure around the [Co(en)3]3+ (en=ethyldiamine) solute.We show that the final spectrum converges quickly with respect to the number of configurations. The DRF results were compared with results obtained from the much simpler conductor-like screening model (COSMO). Both models predicted similar blue shifts of the CD bands, but none of the models is in perfect agreement with the experiments. For instance, the calculated intensities are larger than what is found experimentally if reasonable empirical line width parameters are applied. From the DRF computations, we further show that almost all the solvent effects arise from ground-state solvation. Thus, ignoring the dynamic solvent response is a good approximation for a system like [Co(en)3]3+, where the solute is highly charged and the solvent is very polar.

This is a special issue of the International Journal of Quantum Chemistry, dedicated to the memory of Jaap G. Snijders, Professor of Theoretical Chemistry at the University of Groningen, the Netherlands, until his untimely death on August 13, 2003. In this issue are collected original contributions by his former PhD students and collaborators. In addition, Cederbaum and Streltsov [1] had already published a paper dedicated to his memory in 2003. We are International Journal of Quantum Chemistry, Vol 106, 2409 (2006) 2006 Wiley Periodicals, Inc. sure that Jaap would have loved to read and discuss these papers.

Ria Broer and Joop van Lenthe

Guest Editors

Department of Chemical Physics, Barcelona Science Park, Universitat de Barcelona, C/Marti i Franquès 1, 08028 Barcelona, Spain

Department of Physical and Inorganic Chemistry, Universitat Rovira i Virgili, Marcel.li Domingo s/n, 43007 Tarragona, Spain

We recently developed a scheme for first-principles calculations of hopping matrix elements between localized states in extended systems. We apply the scheme to the determination of double exchange (DE) parameters in lightly hole-doped LaMnO3 and electron-doped CaMnO3. DE is one of the important factors for understanding the properties of doped manganites. The calculations are based on the construction of wave functions for localized hole states or localized electron states for large embedded clusters. The wave functions of these clusters are expressed in terms of localized orbitals, obtained from calculations on smaller units, or "fragments", centered around a transition metal ion. The starting point of electronic states expressed in terms of localized orbital sets is conceptually attractive. It also allows for a rigorous treatment of local electron correlation and electronic relaxation effects. In the present study, the fragments are embedded [MnO6] units. The large clusters contain either two or four Mn ions and all neighboring oxygen ligands. The results are compared with conventional embedded cluster calculations. In both compounds, the effective hopping matrix elements, or "double exchange" (DE) parameters, in the ab planes (in the Pbnm space group) are larger than along the c axes. We found nearly perfect agreement with the Anderson-Hasegawa model for the spin dependence of the DE parameters. Nearest-neighbor parameters are more than one order of magnitude larger than next nearest-neighbor parameters. In LaMnO3 the DE in the ab planes is about -0.26 eV. If there were no Jahn-Teller distortion present in the material, it would have been twice as large. In CaMnO3, the corresponding nearest-neighbor DE parameter for hopping of a doped electron in the ab planes is only about -0.17 eV, due to the antiparallel spin coupling. However, since this interaction is much larger than the exchange coupling, we suggest that it induces local ferromagnetic clusters around the doped electrons.

Letter to the editor

Department of Chemistry, Georgia State UniVersity, P.O. Box 4098, Atlanta, GA, 30302-4098

In this work we present calculated absorption and emission spectra in acetonitrile (MeCN) solution of N-acetyl- 1-aminopyrene (PAAc, a spectroscopic model compound) and N-(1-pyrenyl)-1-methyluracil-5-carboxamide (PAUMe, a computational model for 5-(N-carboxyl-1-aminopyrenyl)-2'-deoxyuridine (PAdU)). The computational method used-the discrete reaction field approach (DRF)-combines a quantum mechanical (QM) description of the solute (here DFT and INDOs/CIS, i.e., the INDO parametrization for spectroscopy) with a classical, molecular mechanics (MM) description of the solvent molecules. The latter are modeled with point charges representing the permanent charge distribution and polarizabilities to account for many-body interactions among the solute and other solvent molecules. Molecular dynamics is used to sample the degrees of freedom of the solution around several solute conformations each in two electronic excited states. This leads to a large number of solute/solvent configurations from which 800 are selected for each excited state and collected into a single ensemble by means of proper Boltzmann averaging. DRF INDOs/CIS applied to the selected solute/solvent configurations give simulated absorption and emission band spectra-each based on 15200 calculated transitions-that compare well with experimental results. For example, the much broader absorption and emission bands in PAdU compared with PAAc are reproduced, and the simulated emission spectra of PAUMe agree well with broad (380-550 nm) charge transfer (CT) emission seen for PAdU in MeCN. The observed multiexponential fluorescence decay profiles for PAdU in different polar solvents are interpreted in terms of solute/solvent conformational heterogeneity here generated in the MD simulations for PAUMe in MeCN. Additionally, the simulations demonstrate the mixing of the forbidden Py+/dU- CT states with allowed pyrenyl 1(,*) states.

In this work we discuss the application of nonequilibrium Green functions theory to atomic and molecular systems with the aim to study charge and energy transport in these systems. We apply the Kadanoff-Baym equations to atoms and diatomic molecules initially in the ground state. The results obtained for the correlated initial states are used to analyze variational energy functionals of the Green function which are shown to perform very well. We further show an application of the Kadanoff-Baym equations to a molecule exposed to an external laser field. Finally we discuss the connection between nonequilibrium Green function theory and time-dependent density-functional theory with the aim to develop better density functionals in order to treat larger systems than those attainable with the nonequilibrium Green function method.

While the use of Green's function techniques has a long tradition in quantum chemistry, the possibility of propagating the Kadanoff-Baym equations has remained largely unexplored. We have implemented the time-propagation for atoms and diatomic molecules, starting from a system in the groundstate. The initial stage of the calculation requires solving the Dyson equation self-consistently for the equilibrium Green's function. This Green's function contains a huge amount of information, and we have found it particularly interesting to compare the self-consistent total energies to the results of variational energy functionals of the Green's function. We also use time-propagation for calculating linear response functions, as a means for obtaining the excitation energies of the system. We have presently implemented the propagation for the second Born approximation, while the GW approximation has now been implemented for the ground state calculations.

Chapter 6 in "Time-Dependent Density Functional Theory"

The coupling between electronic and nuclear motion plays an essential role in a wide range of physical phenomena. A few important research fields in which this is the case are superconductivity in solids, quantum transport where one needs to take into account couplings between electrons and phonons, the polaronic motion in polymer chains, and the ionization-dissociation dynamics of molecules in strong laser fields. Our goal is to set up a time-dependent multicomponent density-functional theory (TDMCDFT) to provide a general framework to describe these diverse phenomena. In TDMCDFT the electrons and nuclei are treated completely quantum mechanically from the outset. The basic variables of the theory are the electron density n, which will be defined in a body-fixed frame attached to the nuclear framework, and the diagonal of the nuclear N-body density matrix I', which will depend on all the nuclear coordinates. The chapter is organized as follows: We start out by defining the coordinate transformations to obtain a suitable Hamiltonian for defining our densities to be used as basic variables in the theory. We then discuss the basic one-to-one correspondence between TD potentials and TD densities, and subsequently, the resulting TD Kohn-Sham equations, the action functional, and linear response theory. As an example we discuss a diatomic molecule in a strong laser field.

Chapter 32 in "Time-Dependent Density Functional Theory"

The nomenclature quantum transport has been coined for the phenomenon of electron motion through constrictions of transverse dimensions smaller than the electron wavelength, e.g., quantum-point contacts, quantum wires, molecules, etc. To describe transport properties on such a small scale, a quantum theory of transport is required. In this Chapter we focus on quantum transport problems whose experimental setup is schematically displayed in Fig. 32.1a. A central region of meso- or nanoscopic size is coupled to two metallic electrodes which play the role of charge reservoirs. The whole system is initially in a well defined equilibrium configuration, described by a unique temperature and chemical potential (thermodynamic consistency). No current flows through the junction, the charge density of the electrodes being perfectly balanced. In the previous Chapter, Gebauer et al. proposed to join the left and right remote parts of the system so to obtain a ring geometry, see Fig. 30.1. In their approach the electromotive force is generated by piercing the ring with a magnetic field that increases linearly in time. Here, we consider the longitudinal geometry of Fig. 32.1a and describe an alternative approach. As originally proposed by Cini [Cini 1980], we may drive the system out of equilibrium by exposing the electrons to an external time-dependent potential which is local in time and space. For instance, we may switch on an electric field by putting the system between two capacitor plates far away from the system boundaries, see Fig. 32.1b. The dynamical formation of dipole layers screens the potential drop along the electrodes and the total potential turns out to be uniform in the left and right bulks. Accordingly, the potential drop is entirely limited to the central region. As the system size increases, the remote parts are less disturbed by the junction, and the density inside the electrodes approaches the equilibrium bulk density.

Chapter 2 in "Time-Dependent Density Functional Theory"

The Runge-Gross theorem [Runge 1984] states that for a given initial state the time-dependent density is a unique functional of the external potential. Let us elaborate a bit further on this point. Suppose we could solve the timedependent Schrodinger equation (TDSE) for a given many-body system, i.e., we specify an initial state |0 at t = t0 and evolve the wave function in time using the Hamiltonian H (t). Then, from the wave function, we can calculate the time-dependent density n(r, t). We can then ask the question whether exactly the same density n(r, t) can be reproduced by an external potential v ext(r, t) in a system with a different given initial state and a different two-particle interaction, and if so, whether this potential is unique (modulo a purely time-dependent function). The answer to this question is obviously of great importance for the construction of the time-dependent Kohn-Sham equations. The Kohn-Sham system has no two-particle interaction and differs in this respect from the fully interacting system. It has, in general, also a different initial state. This state is usually a Slater determinant rather than a fully interacting initial state. A time-dependent Kohn-Sham system therefore only exists if the question posed above is answered affirmatively. Note that this is a v-representability question: Is a density belonging to an interacting system also noninteracting v-representable? We will show in this chapter that, with some restrictions on the initial states and potentials, this question can indeed be answered affirmatively [van Leeuwen 1999, van Leeuwen 2001, Giuliani 2005]. We stress that we demonstrate here that the interacting-v-representable densities are also noninteracting-v-representable rather than aiming at characterizing the set of v-representable densities. The latter question has inspired much work in ground state density functional theory (for extensive discussion see [van Leeuwen 2003]) and has only been answered satisfactorily for quantum lattice systems [Chayes 1985].

Chapter 3 in "Time-Dependent Density Functional Theory"

In this chapter we give an introduction to the Keldysh formalism, which is an extremely useful tool for first-principles studies of nonequilibrium manyparticle systems. Of particular interest for TDDFT is the relation to nonequilibrium Green functions (NEGF), which allows us to construct exchangecorrelation potentials with memory by using diagrammatic techniques. For many problems, such as quantum transport or atoms in intense laser pulses, one needs exchange-correlation functionals with memory, and Green function techniques offer a systematic method for developing these. The Keldysh formalism is also necessary for defining response functions in TDDFT and for defining an action functional needed for deriving TDDFT from a variational principle. In this chapter, we give an introduction to the nonequilibrium Green function formalism, intended to illustrate the usefulness of the theory. The formalism does not differ much from ordinary equilibrum theory, the main difference being that all time-dependent functions are defined for time-arguments on a contour, known as the Keldysh contour.

Chapter 21 in "Time-Dependent Density Functional Theory"

In general, finite field DFT and TDDFT calculations yield accurate values for the response properties of molecular systems when standard approximations for the exchange-correlation functionals are used [Gross 1996]. In combination with their high efficiency, this makes these theoretical approaches ideal candidates for the calculation of physical properties of large molecular systems of technological interest. For an important class of materials, however, this potential is not yet realized.

Chapter 19 in "Time-Dependent Density Functional Theory"

The description of the ground state of crystalline systems within density functional theory, and of their response to external fields within the timedependent version of this theory, relies heavily on the use of periodic boundary conditions. As a model for the bulk part of the system one considers a large region containing N elementary unit cells. Then, while imposing constraints that ensure the single-valuedness and periodicity of the wave function at the boundary, one considers the limit of infinite N to derive properties for the macroscopic samples. In this treatment, one implicitly assumes that the Hohenberg-Kohn theorem [Hohenberg 1964] and the Kohn-Sham approach [Kohn 1965], and their time-dependent equivalents derived by Runge and Gross [Runge 1984], apply separately to the bulk part of the system. This implies that effects caused by density changes at the outer surface, which are artificially removed in this periodic boundary approach, can be neglected. However, this can not be justified as these effects are real.

Chapter 13 in "Time-Dependent Density Functional Theory"

The induced density can be obtained within a linear response calculation by solving a coupled set of equations, in which the first order change in the density follows from the first order change in the self-consistent potential and vice versa.

The geometry of a molecule or solid determines many of its physical and chemical properties. The ground state geometry of any system can be found by a geometry optimization. This procedure can also be used for e.g. transition state searches, reactions at surfaces etc. In the Born-Oppenheimer (BO) approximation, the total ground state energy of a system is a function of the coordinates of all nuclei (E0 = E0({R})). The minimum of the energy corresponds to the ground state geometry, whereas a first order saddlepoint on the BO-surface gives the transition state geometry. In solids we can vary two kinds of parameters; the nuclear coordinates of the atoms in the unit cell and the unit cell parameters. In this report, we will only consider the nuclear coordinates in a fixed unit cell. In order to find the stationary point of interest, we need the gradients of the energy with respect to the nuclear coordinates. These could be calculated by a numerical interpolation, but this is computationally not feasible, even for small systems. In this report, we give an analytical expression for the gradient of the total Kohn-Sham energy with respect to the nuclear coordinates.

The Direct Reaction Field (DRF) approach has proven to be a useful tool to investigate the influence of solvents on the quantum/classical behaviour of solute molecules. In this paper, we report the latest extension of this DRF approach, which consists of the gradient of the completely classical energy expressions of this otherwise QM/MM method. They can be used in (completely classical) Molecular Dynamics simulations and geometry optimizations, that can be followed by a number of single point QM/MM calculations on configurations obtained in these simulations/optimizations. We report all energy and gradient expressions, and results for a number of interesting (model) systems. They include geometry optimization of the benzene dimer as well as Molecular Dynamics simulations of some solvents. The most stable configuration for the benzene dimer is shown to be the parallel-displaced form, which is slightly more stable (0.3 kcal/mol) than the T-shaped dimer.

Recently the linear response of metallic solids has been formulated within the time-dependent current-density-functional approach [Romaniello and de Boeij, Phys. Rev. B 71, 155108 (2005)]. The implementation, which originally used only the adiabatic local density approximation for the exchange-correlation kernel is extended in order to include also the Vignale-Kohn current functional. Within this approximation the exchange-correlation kernel is frequency dependent, thus relaxation effects due to electron-electron scattering can now be taken into account and some deficiencies of the adiabatic local density approximation (ALDA), as the absence of the low-frequency Drude-like tail in absorption spectra, can be cured. We strictly follow the previous formulation of the linear response of semiconductors by using the Vignale-Kohn functional [Berger, de Boeij, and van Leeuwen, Phys. Rev. B 71, 155104 (2005)]. The self-consistent equations for the interband and intraband contributions to the induced density and current density, which are completely decoupled within the ALDA and in the long-wavelength limit, now remain coupled. We present our results calculated for the optical properties of the noble metals Cu, Ag, and Au and we compare them with measurements found in literature. In the case of Au we treat the dominant scalar relativistic effects using the zeroth-order regular approximation in the ground-state density-functional-theory calculations, as well as in the time-dependent response calculations.

Theoretical Chemistry, Rijksuniversiteit Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands

Laboratory of Physics, Helsinki University of Technology - TKK, P.O.Box 1100, FI-02015 TKK, Finland

Institute of Mathematics, Helsinki University of Technology - TKK, P.O. Box 1100, FI-02015 TKK, Finland

Institute of Physics, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere, Finland

Mathematics Division, Faculty of Technology, University of Oulu, P.O.Box 4500, FI-90014 Finland

Institut für Theoretische Physik, Johannes Kepler Universität, A-4040 Linz, Austria

A characteristic feature of the state-of-the-art of real-space methods in electronic structure calculations is the diversity of the techniques used in the discretization of the relevant partial differential equations. In this context, the main approaches include finite-difference methods, various types of finite-elements and wavelets. This paper reports on the results of several code development projects that approach problems related to the electronic structure using these three different discretization methods. We review the ideas behind these methods, give examples of their applications, and discuss their similarities and differences.

Department of Physics, Lund University, Sölvegatan 14 A, SE-22362 Lund, Sweden

We have calculated total energies of atoms and diatomic molecules from the Luttinger-Ward functional, using self-energy approximations to second order as well as the GW approximation. In order to assess the variational quality of this functional, we have also solved the Dyson equation self-consistently. The Luttinger- Ward functional is compared to the variational functional due to Klein, and we demonstrate that the variational property of the latter functional is inferior to that of the Luttinger-Ward functional. We also show how to obtain variational density functionals from the functionals of the Green function. These orbital functional schemes are important for systems where density-functional theory using local functionals of the density necessarily fails. We derive an optimized effective potential OEP scheme that is based on the Luttinger-Ward functional and, unlike the conventional OEP schemes, produces energies in good agreement with the values obtained from the self-consistent Green function. Our calculations show that, when applied to molecules, the Luttinger-Ward functional is more sensitive to the quality of the input Green function than when applied to atoms, but the energies are remarkably close to the self-consistent values when the Hartree-Fock Green function is used as input. This Luttinger-Ward functional is therefore a simple and efficient method for studying the merits of various self-energy approximations while avoiding the computationally demanding task of solving the Dyson equation self-consistently.

Département de Physique de la Matière Condensée, Université de Genève, CH-1211 Genève 4, Switzerland

We present a systematic analysis of the optical properties of bcc transition metals in the groups VB: V, Nb and Ta, and VIB: paramagnetic Cr, Mo and W. For this we use our formulation of time-dependent current-density-functional theory for the linear response of metals. The calculated dielectric and electron energy loss functions are compared with new ellipsometry measurements and with data reported in literature, showing an overall good agreement. The experimental data of the dielectric functions presented by Nestell and Christy and by Weaver et al. differ mostly in the low-frequency region. However we found that their reflectivity data are in very good agreement up to about 3 eV. We attribute this apparent discrepancy to the Drude-like extrapolation model used by Weaver et al. in the Kramers-Kronig procedure to extract the optical constants from their reflectivity data. Our experiments are in good agreement with Nestell and Christy's data. The calculated absorption spectra show some deviations from the experiments, in particular in the 3d metals. We assign the spectra in terms of transitions between pairs of bands and we analyze which parts of the Brillouin zone are mainly involved in the absorption. Our results suggest that the blue-shift of some spectral features in our calculations can be attributed mainly to the incorrect description of the virtual d-bands by the approximations used for the ground state exchange-correlation functional. These virtual bands are too weakly bound by the local density and generalized gradient approximations, in particular in the 3d metals. We calculate separately the inter- and intraband contributions to the absorption and we show using a k⋅p analysis that, within the scalar-relativistic approximation, interband transitions contribute to the absorption already at frequencies well below 0.5 eV. This finding makes questionable the Drude-like behavior normally assumed in the experimental analysis of the linear response. We find that the combination of the Drude model in which we use the calculated plasma frequency and an optimized relaxation time, and the calculated interband response can well describe the experimental spectra. The electron energy loss spectra are very well reproduced by our calculations showing in each metal a dominant plasmon peak at about 22-24 eV, well above the corresponding Drude-like free-electron plasma frequency, and additional features in the range 10-15 eV. We show that the renormalization of the plasma frequency is due to the interplay between inter- and intraband processes, and that the additional features arise from the rich structure in the dielectric function caused by interband transitions.

We derive variational expressions for the grand potential or action in terms of the many-body Green function G which describes the propagation of particles and the renormalized four-point vertex which describes the scattering of two particles in many-body systems. The main ingredient of the variational functionals is a term we denote as the Ξ-functional which plays a role analogously to the usual Φ-functional studied by Baym (G.Baym, Phys.Rev. 127, 1391 (1962)) in connection with the conservation laws in many-body systems. We show that any Ξ-derivable theory is also Φ-derivable and therefore respects the conservation laws. We further set up a computational scheme to obtain accurate total energies from our variational functionals without having to solve computationally expensive sets of self-consistent equations. The input of the functional is an approximate Green function \tilde {G} and an approximate four-point vertex \tilde {Γ} obtained at a relatively low computational cost. The variational property of the functional guarantees that the error in the total energy is only of second order in deviations of the input Green function and vertex from the self-consistent ones that make the functional stationary. The functionals that we will consider for practical applications correspond to infinite order summations of ladder and exchange diagrams and are therefore particularly suited for applications to highly correlated systems. Their practical evaluation is discussed in detail.

Polymers were known to be good insulators for a long time. Almost three decades ago another type of polymers was discovered, the conducting polymers. The first conducting polymer found was polyacetylene which was doped with iodine[1]. The electrical conductivity found for this sample was found to be in the order of metallic conductivity, which is about fifteen orders of magnitude larger than for insulators. H. Shirakawa, A.J. Heeger and A.G. MacDiarmid even received the Nobel Prize for their discovery. It did not take long after this discovery before new polymers were found to be good conductors. An essential property of the conducting polymers is the fact that they are all conjugated polymers. Because of the fact that electrons are being delocalized makes the polymer suitable to transport charge through the polymer chains. If one is able to control the conducting properties in a good way many possible applications can be thought of. Polymeric electronic wires may substitute the inorganic ones used nowadays in the chip industry. The thickness of the wire, lying in the order of 1 molecule, is a good progress. Other applications are Light Emitting Diodes (LEDs), photovoltaic cells, photodetectors and optocouplers are some applications that have already been fabricated. These applications are possible due to the fact that a bandgap is present in conjugated polymers. The theoretical description of the conducting properties has been subject of a lot of discussion. Especially the relation between electron-electron interactions and electron-phonon interactions is still being investigated. Lots of properties have been explained by focusing on the electron-phonon interactions. This essay also shows the advantage of focusing on electron-electron interactions. At last I will give my point of view on the current state of discussions.

In this paper it is argued that the use of density functional theory (DFT) to solve the exact, non-relativistic, many-electron problem, for magnetic systems requires imposing space and spin symmetry constraints exactly in the same way as it is currently done in ab initio wave function theory. This strong statement is supported on pertinent calculations for selected systems representative of organic diradicals, molecular magnets and antiferromagnetic solids. These calculations include several wave function methods of increasing accuracy and different forms of the exchange-correlation functional. The comparisons of numerical results carried out always within the same standard Gaussian Type Orbital atomic basis set show that imposing or not the spin and space constraints (restricted or unrestricted formalisms) leads to contradictory results. Therefore, it appears that, in the case of the Heisenberg magnetic constant, the present exchange-correlation functionals may provide reasonable numerical results although for the wrong physical reasons thus evidencing the failure of the current DFT methods to properly describe magnetic systems.

Department of Organic and Inorganic Chemistry, Free University de Boelelaan 1083, 1081 HV Amsterdam

Department of Theoretical Chemistry, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen

Groningen Biomolecular Sciences and Biotechnology Institute

Department of Biopysical Chemistry, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen

Swiss Federal Institute of Technology, ETH Hönggerberg, 8093 Zürich

Azurin from Pseudomonas aeruginosa is a small 128-residue, copper-containing protein. Its redox potential can be modified by mutating the protein. Free-energy calculations based on classical molecular-dynamics simulations of the protein and from mutants in aqueous solution at different pH values were used to compute relative redox potentials. The precision of the free-energy calculations with the λ coupling-parameter approach is evaluated as function of the number and sequence of λ values, the sampling time and initial conditions. It is found that the precision is critically dependent on the relaxation of hydrogen-bonding networks when changing the atomic-charge distribution due to a change of redox state or pH value. The errors in the free energies range from 1 to 10 kBT, depending on the type of process. Only qualitative estimates of the change in redox potential by protein mutation can be obtained.

No abstract available

We have calculated the frequency-dependent refractive index and the third-order nonlinear susceptibility for C60 in the condensed phase, which is related to third-harmonic generation (THG) and degenerate four-wave mixing (DFWM) experiments. This was done using the recently developed discrete solvent reaction field (DRF) model, which combines a time-dependent density functional theory (TD-DFT) description of the central C60 molecule with a classical polarizable MM model for the rest of the fullerene cluster. Using this model, effective microscopic properties can be calculated that, combined with calculated local field factors, give macroscopic susceptibilities. The largest calculation was for a cluster of 63 C60 molecules in which the central molecule was treated with TD-DFT. For this molecule, the effective polarizability was increased with about 15% and the effective second hyperpolarizability with about 60% compared with the gas phase. The calculated refractive index was found to be in good agreement with experiments and other theoretical results. The agreement with THG experiments was within a factor of two, whereas for DFWM the agreement was less good due to the neglect of vibrational contributions in the calculations. It was found that it is more important to account for the dispersion in the third-order susceptibilities than in the corresponding second hyperpolarizability.

Division Solid State Theory, Department of Physics, Lund University, Sölvegatan 14A, S-22362, Lund, Sweden

It was recently proposed to use variational functionals based on many-body perturbation theory for the calculation of the total energies of many-electron systems. The accuracy of such functionals depends on the degree of sophistication of the underlying perturbation expansions. The energy functionals are variational in the sense that they can be evaluated at rather crude approximations to their independent variables, which are the one-electron Green function, or the one-electron Green function and the dynamically screened electron interaction. The functionals were previously applied to the electron gas and shown to be extraordinarily accurate already at the level of the so-called GW approximation (GWA). In the present work we have tested the functional due to Luttinger and Ward, which is a functional of the Green function. Using DFT and Hartree-Fock Green functions as input variables, we have calculated total energies of diatomic molecules at the level of the GWA as well as with second-order exchange effects included.We will also discuss various other variational energy functionals, including DFT orbital functionals based on many-body perturbation theory.

Theoretical Chemistry, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

A polarizable quantum mechanics and molecular mechanics model has been extended to account for the difference between the macroscopic electric field and the actual electric field felt by the solute molecule. This enables the calculation of effective microscopic properties which can be related to macroscopic susceptibilities directly comparable with experimental results. By seperating the discrete local field into two distinct contribution we define two different microscopic properties, the so-called solute and effective properties. The solute properties account for the pure solvent effects, i.e., effects even when the macroscopic electric field is zero, and the effective properties account for both the pure solvent effects and the effect from the induced dipoles in the solvent due to the macroscopic electric field. We present results for the linear and nonlinear polarizabilities of water and acetonitrile both in the gas phase and in the liquid phase. For all the properties we find that the pure solvent effect increases the properties whereas the induced electric field decreases the properties. Furthermore, we present results for the refractive index, third-harmonic generation (THG), and electric field induced second-harmonic generation (EFISH) for liquid water and acetonitrile. We find in general good agreement between the calculated and experimental results for the refractive index and the THG susceptibility. For the EFISH susceptibility, however, the difference between experiment and theory is larger since the orientational effect arising from the static electric field is not accurately described.

Theoretische Chemie, Vrije UniVersiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands,

and Theoretical Chemistry, Materials Science Center, UniVersity of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

In this paper the role of the solvent in the formation of the charge-separated excited state of 9,9'-bianthryl (BA) is examined by means of mixed molecular mechanical/quantum mechanical (QM/MM) calculations. It is shown that in weakly polar solvents a relaxed excited state is formed with an interunit angle that is significantly smaller than 90°. This relaxed excited state has a considerable dipole moment even in weakly polar solvents; for benzene and dioxane dipole moments of ca. 6 D were calculated, which is close to experimental data. These dipoles are induced by the solvent in the highly polarizable relaxed excited state of BA, and the dipole relaxation time is governed by solvent reorganizations. In polar solvent the charge separation is driven to completion by the stronger dipoles in the solvent and a fully charged separated excited state is formed with an interunit angle of 90°.

We have calculated the self-consistent Green's function for a number of atoms and diatomic molecules. This Green's function is obtained from a conserving self-energy approximation, which implies that the observables calculated from the Green's functions agree with the macroscopic conservation laws for particle number, momentum, and energy. As a further consequence, the kinetic and potential energies agree with the virial theorem, and the many possible methods for calculating the total energy all give the same result. In these calculations we use the finite temperature formalism and calculate the Green's function on the imaginary time axis. This allows for a simple extension to nonequilibrium systems. We have compared the energies from self-consistent Green's functions to those of nonselfconsistent schemes and also calculated ionization potentials from the Green's functions by using the extended Koopmans' theorem.

We included relativistic effects in the formulation of the time-dependent current-density-functional theory for the calculation of linear response properties of metals [P. Romaniello and P. L. de Boeij, Phys. Rev. B (to be published)]. We treat the dominant scalar-relativistic effects using the zeroth-order regular approximation in the ground-state density-functional theory calculations, as well as in the time-dependent response calculations. The results for the dielectric function of gold calculated in the spectral range of 0-10 eV are compared with experimental data reported in literature and recent ellipsometric measurements. As well known, relativistic effects strongly influence the color of gold. We find that the onset of interband transitions is shifted from around 3.5 eV, obtained in a nonrelativistic calculation, to around 1.9 eV when relativity is included. With the inclusion of the scalar-relativistic effects there is an overall improvement of both real and imaginary parts of the dielectric function over the nonrelativistic ones. Nevertheless some important features in the absorption spectrum are not well reproduced, but can be explained in terms of spin-orbit coupling effects. The remaining deviations are attributed to the underestimation of the interband gap (5d-6sp band gap) in the local-density approximation and to the use of the adiabatic local-density approximation in the response calculation.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

In this work we have investigated the first hyperpolarizability of pNA in 1,4-dioxane solution using a quantum mechanics/molecular mechanics QM/MM model. The particular model adopted is the recently developed discrete solvent reaction field DRF model. The DRF model is a polarizable QM/MM model in which the QM part is treated using time-dependent density-functional theory and local-field effects are incorporated. This allows for direct computation of molecular effective properties which can be compared with experimental results. The solvation shift for the first hyperpolarizability is calculated to be 30% which is in good agreement with the experimental results. However, the calculated values, both in the gas phase and in solution, are by a factor of 2 larger than the experimental ones. This is in contrast to the calculation of the first hyperpolarizability for several small molecules in the gas phase where fair agreement is found with experimental. The inclusion of local-field effects in the calculations was found to be crucial and neglecting them led to results which are significantly larger. To test the DRF model the refractive index of liquid 1,4-dioxane was also calculated and found to be in good agreement with experiment.

A simple modification of the zeroth-order regular approximation (ZORA) in relativistic theory is suggested to suppress its erroneous gauge dependence to a high level of approximation. The method, coined gauge-independent ZORA (ZORA-GI), can be easily installed in any existing nonrelativistic quantum chemical package by programming simple one-electron matrix elements for the quasirelativistic Hamiltonian. Results of benchmark calculations obtained with ZORA-GI at the Hartree-Fock (HF) and second-order Mller-Plesset perturbation theory (MP2) level for dihalogens X_{2} (X=F,Cl,Br,I,At) are in good agreement with the results of four-component relativistic calculations (HF level) and experimental data (MP2 level). ZORA-GI calculations based on MP2 or coupled-cluster theory with single and double perturbations and a perturbative inclusion of triple excitations [CCSD(T)] lead to accurate atomization energies and molecular geometries for the tetroxides of group VIII elements. With ZORA-GI/CCSD(T), an improved estimate for the atomization energy of hassium (Z=108) tetroxide is obtained.

The regular approximation to the normalized elimination of the small component (NESC) in the modified Dirac equation has been developed and presented in matrix form. The matrix form of the infinite-order regular approximation (IORA) expressions, obtained in [Filatov and Cremer, J. Chem. Phys. 118, 6741 (2003)] using the resolution of the identity, is the exact matrix representation and corresponds to the zeroth-order regular approximation to NESC (NESC-ZORA). Because IORA (=NESC-ZORA) is a variationally stable method, it was used as a suitable starting point for the development of the second-order regular approximation to NESC (NESC-SORA). As shown for hydrogenlike ions, NESC-SORA energies are closer to the exact Dirac energies than the energies from the fifth-order Douglas-Kroll approximation, which is much more computationally demanding than NESC-SORA. For the application of IORA (=NESC-ZORA) and NESC-SORA to many-electron systems, the number of the two-electron integrals that need to be evaluated sidentical to the number of the two-electron integrals of a full DiracHartreeFock calculationd was drastically reduced by using the resolution of the identity technique. An approximation was derived, which requires only the two-electron integrals of a nonrelativistic calculation. The accuracy of this approach was demonstrated for heliumlike ions. The total energy based on the approximate integrals deviates from the energy calculated with the exact integrals by less than 5x10^{-9} hartree units. NESC-ZORA and NESC-SORA can easily be implemented in any nonrelativistic quantum chemical program. Their application is comparable in cost with that of nonrelativistic methods. The methods can be run with density functional theory and any wave function method. NESC-SORA has the advantage that it does not imply a picture change.

Department of Chemistry and Department of Physics, University of the Pacific, 3601 Pacific Avenue, Stockton, California 95211-0110

It is demonstrated that the LYP correlation functional is not suited to be used for the calculation of electron spin resonance hyperfine structure (HFS) constants, nuclear magnetic resonance spin-spin coupling constants, magnetic, shieldings and other properties that require a balanced account of opposite- and equal-spin correlation, especially in the core region. In the case of the HFS constants of alkali atoms, LYP exaggerates opposite-spin correlation effects thus invoking too strong in-out correlation effects, an exaggerated spin-polarization pattern in the core shells of the atoms, and, consequently, too large HFS constants. Any correlation functional that provides a balanced account of opposite- and equal-spin correlation leads to improved HFS constants, which is proven by comparing results obtained with the LYP and the PW91 correlation functional. It is suggested that specific response properties are calculated with the PW91 rather than the LYP correlation functional.

Department of Physics, Kaiserslautern University, Box 3049, D-67653, Kaiserslautern, Germany

We show the results of ab initio embedded cluster calculations of the ground state and lowlying excited states of the (001) surface and bulk of NiO including the spin-orbit coupling effects. The calculations are performed by the Columbus package using the combination of the relativistic effective core potentials and the spin-orbit operators. These effects result in the splitting of the d-d excited states. The fine structure of the 3d^{8}; levels of the Ni^{2+} ion in NiO bulk and its (001) surface is resolved yielding good agreement with experimentally observed second-harmonic and optical absorption spectra. In addition, we discuss the transition electric-dipole moments, which can be used for a quantitative comparison with the experimentally determined optical intensities as well as for the exploration of various ultrafast all-optical spin-switching scenario.

In this work we employ the Vignale-Kohn sVKd current functional in the calculation of the linear response properties of polyacetylene for both the one-dimensional infinite chain and the infinite three-dimensional crystal. We test the two existing parametrizations of the longitudinal and transverse exchange-correlation kernels of the homogeneous electron gas that enter the VK functional and show that they lead to very different results. We argue that this is mainly caused by the different values of these kernels in the zero-frequency limit in the two parametrizations. In this limit knowledge of the exchange correlation part of the shear modulus of the homogeneous electron gas becomes very important. It is exactly this quantity that is not known accurately. Furthermore, we show that our results are in good qualitative agreement with results obtained earlier using the Vignale-Kohn functional for polyacetylene oligomers.

We extend the formulation of time-dependent current-density-functional theory for the linear response properties of dielectric and semi-metallic solids [Kootstra, J. Chem. Phys. 112, 6517 (2000)] to treat metals as well. To achieve this, the Kohn-Sham response functions have to include both interband and intraband transitions with an accurate treatment of the Fermi surface in the Brillouin-zone integrations. The intraband contributions in particular have to be evaluated using a wave-vector-dependent description. To test the method we calculate the optical properties of the two noble metals Cu and Ag. The dielectric and energy loss functions are compared with experiments and with the classical Drude theory. In general we find a good agreement with the experiments for the calculated results obtained within the adiabatic local density approximation. In order to describe the Drude-like absorption below the interband onset and the sharp plasma feature in silver exchange-correlation, effects beyond the adiabatic local density approximation are needed, which may be included in a natural way in the present current-density-functional approach.

Rijkuniversiteit Groningen, Theoretical Chemistry, Materials Science Center, 9747AG, Nijenborgh 4, Groningen, The Netherlands

In the present work, we propose a theory for obtaining successively better approximations to the linear response functions of time-dependent density or current-density functional theory. The new technique is based on the variational approach to many-body perturbation theory MBPT as developed during the sixties and later expanded by us in the mid-nineties. Due to this feature, the resulting response functions obey a large number of conservation laws such as particle and momentum conservation and sum rules. The quality of the obtained results is governed by the physical processes built in through MBPT but also by the choice of variational expressions. We here present several conserving response functions of different sophistication to be used in the calculation of the optical response of solids and nanoscale systems.

The aim of this work is to provide a physical model to relate the polarizability per unit cell of oligomers to that of their corresponding infinite polymer chains. For this we propose an extrapolation method for the polarizability per unit cell of oligomers by fitting them to a physical model describing the dielectric properties of polymer chains. This physical model is based on the concept of a dielectric needle in which we assume a polymer chain to be well described by a cylindrically shaped nonconducting rod with a radius much smaller than its length. With this model we study in which way the polarizability per unit cell approaches the limit of the infinite chain. We show that within this model the macroscopic contribution of the induced electric field to the macroscopic electric field vanishes in the limit of an infinite polymer chain, i.e., there is no macroscopic screening. The macroscopic electric field becomes equal to the external electric field in this limit. We show that this identification leads to a relation between the polarizability per unit cell and the electric susceptibility of the infinite polymer chain. We test our dielectric needle model on the polarizability per unit cell of oligomers of the hydrogen chain and polyacetylene obtained earlier using time-dependent current-density-functional theory in the adiabatic local-density approximation and with the Vignale-Kohn functional. We also perform calculations using the same theory on truly infinite polymer chains by employing periodic boundary conditions. We show that by extrapolating the oligomer results according to our dielectric needle model we get good agreement with our results from calculations on the corresponding infinite polymer chains. 2005 American Institute of Physics.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Pacific Northwest National Laboratory, 902 Battelle Boulevard, P.O. Box 999, Richland, WA 99352

A previously neglected intra-atomic many-body effect has important consequences for the X-ray photoelectron spectra (XPS) of transition metal atoms and cations. This effect involves configurations where one elctron is promoted to a 4f orbital and another is dropped to fill the XPS hole; this can be viewed as a frustrated Auger configuration (FAC). The identification of this FAC is a major advance in the understanding of many-body effects in XPS. Its use affects the multiplet splitting and the absolute binding energy; it can also lead to new satallite structure. Furthermore, it is expected to be generally important.

We present a study of the static polarizability for the tubular fullerenes C(60+10i), where i=0-5, and the closely related [5,5] carbon nanotube, using time-dependent (current)-density-functional theory. Comparing the results obtained within the conventional adiabatic local-density approximation with those obtained using the Vignale-Kohn current-dependent exchange-correlation functional it is found that the extra long-range exchange-correlation effects described by the current-density functional are important to consider, especially for the longest fullerenes. The largest reductions upon inclusion of the resulting counteracting field were found for the longitudinal component of the polarizability, amounting to 18% for the total value of C(110) and about 32% for the value per unit cell in the infinite [5,5] carbon nanotube. For all systems studied the current-density functional results are in good agreeement with experiment, and the agreement with available ab initio self-consistent-field results and results from a point-dipole interaction model is much better than when using the adiabatic local-density functional.

Departament de Quimica Organica Biologica, Institut d'Investigacions Quimiques i Ambientals de Barcelona, I.I.Q.A.B.-CSIC, Jordi Girona 18, 08034-Barcelona, Catalunya, Spain

Density functional theory (DFT) predicts that bicyclo[3.1.0]hexatriene (**2**) is more stable than its isomer *m*-benzyne (**1**). Hess [Eur. J. Org. Chem. (2001) 2185] has argued that experimental findings suggesting **1** can equally or even better be associated with **2**. However, high level ab initio calculations (CCSD(T), CASPT2) show that **2** does not exist and that the previously measured infrared spectrum is correctly assigned to **1**. Bond stretch isomers are possible for *p*-benzynes but not for *m*-benzynes. The electrophilic character of *m*-benzynes is in line with **1** but not with **2**.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Institut für Theoretische Physik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany

We study the dynamics of the electronic and nuclear degrees of freedom for molecules in strong laser fields using an ansatz for the wavefunction that explicitly incorporates the electron-nuclear correlation. Equations of motion for this wavefunction are derived on the basis of the stationary action principle. The method is tested on a one-dimensional model of the H2+ molecule that can be solved essentially exactly by numerical integration of the time-dependent Schrödinger equation. By comparison with this exact solution we find that the correlated approach improves significantly on a mean-field treatment, especially for laser fields strong enough to cause substantial dissociation. These results are very promising since our method has a simple orbital structure and can hence be applied to realistic many-electron molecules.

Mercury chalcogenides HgE (E=O, S, Se, etc.) are described in the literature to possess rather stable bonds with bond dissociation energies between 53 and 30 kcal mol^{-1}, which is actually difficult to understand in view of the closed-shell electron configuration of the Hg atom in its ground state (...4f^{14}5d^{10}6s^{2}). Based on relativistically corrected many body perturbation theory and coupled-cluster theory [IORAmm/MP4, Feenberg-scaled IORAmm/ MP4, IORAmm/CCSD(T)] in connection with IORAmm/B3LYP theory and a [17s14p9d5f]/aug-cc-pVTZ basis set, it is shown that the covalent HgE bond is rather weak (27 kcal mol^{-1}), the ground state of HgE is a triplet rather than a singlet state, and that the experimental bond dissociation energies have been obtained for dimers (or mixtures of monomers, dimers, and even trimers) Hg_{2}E_{2} rather than true monomers. The dimers possess association energies of more than 100 kcalmol^{-1} due to electrostatic forces between the monomer units. The covalent bond between Hg and E is in so far peculiar as it requires a charge transfer from Hg to E (depending on the electronegativity of E) for the creation of a single bond, which is supported by electrostatic forces. However, σ bonding between Hg and E is reduced by strong lone pairlone pair repulsion to a couple of kcalmol^{-1}. Since a triplet configuration possesses somewhat lower destabilizing lone pair energies, the triplet state is more stable. In the dimer, there is a Hg-Hg π bond of bond order 0.66 without any s support. Weak covalent Hg-O interactions are supported by electrostatic bonding. The results for the mercury chalcogenides suggests that all experimental dissociation energies for group-12 chalcogenides have to be revised because of erroneous measurements.

No abstract available

no abstract available

We give a brief overview of nonequilibrium Green function theory and some connections with time-dependent density functional theory (TDDFT). We will focus on how to obtain approximations that satisfy the conservation laws. The account given here is not meant to be comprehensive but tries to put in logical order the main arguments and results that are sometimes found scattered in the literature.

In recent years there have been some rather successful applications of a new variational technique for calculating the total energies of electronic systems. The new method is based on many-body perturbation theory and uses the one-electron Green function as the basic "variable" rather than the wave function of traditional variational calculations. It is the purpose of the present work to promote the new methods within the realm of traditional theoretical chemistry by demonstrating their utility for calculating the correlation energies of a number of atoms at a level corresponding to second-order Møller-Plesset perturbation theory. The generalization to any desired order of perturbation theory is not hard to accomplish.

Placa Imperial Tarraco 1, Tarragona 43005, Spain

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Magnetic interactions in ladder vanadates are determined with quantum chemical computational schemes using the embedded cluster model approach to represent the material. The available experimental data for calcium vanadate is accurately reproduced and the nature of the interladder interaction is established to be ferromagnetic. An analysis of the main contributions to the magnetic couplings is presented and the role of the covalently bonded apex oxygen is elucidated. In the sodium vanadate, the ground state configuration of the rungs is V 3*d ^{1}* - O 2

Erratum: "Excitation energies for a benchmark set of molecules obtained within time-dependent current-density functional theory using the Vignale-Kohn functional" [J. Chem. Phys. 120, 8353 (004)]

In this article we explain how the existing linear response theory of time-dependent density-functional theory can be extended to obtain excitation energies in the framework of time-dependent current-density-functional theory. We use the Vignale-Kohn current-functional [G. Vignale and W. Kohn, Phys. Rev. Lett. 77, 2037 (1996)] which has proven to be successful for describing ultranonlocal exchange-correlation effects in the case of the axial polarizability of molecular chains [M. van Faassen, P. L. de Boeij, R. van Leeuwen, J. A. Berger, and J. G. Snijders, Phys. Rev. Lett. 88, 186401 (2002); J. Chem. Phys. 118, 1044 (2003)]. We study a variety of singlet excitations for a benchmark set of molecules. The * transitions obtained with the Vignale-Kohn functional are in good agreement with experimental and other theoretical results and they are in general an improvement upon the adiabatic local density approximation. In case of the * n transitions the Vignale-Kohn functional fails, giving results that strongly overestimate the experimental and other theoretical results. The benchmark set also contains some other types of excitations for which no clear failures or improvements are observed.

Department of Theoretical Chemistry, Groningen University, Nijenborgh 4 9747 AG Groningen, The Netherlands

Laboratory of Physical Chemistry, Swiss Federal Institute of Technology, Zürich CH-8093, Zürich, Switzerland

Molecular dynamics (MD) simulations have been performed on quercetin 2,3 dioxygenase (2,3QD) to study the mobility and flexibility of the substrate cavity. 2,3QD is the only firmly established Cu-containing dioxygenase known so far. It catalyses the breakage of the O-heterocycle of flavonols. The substrates occupy a shallow and overall hydrophobic cavity proximal to the metal centre of the homo-dimeric enzyme. The linker connecting the C-terminal and N-terminal domains in the monomer is partly disordered in the crystal structure and part of it forms a flexible lid at the entrance of the substrate cavity. This loop has been tentatively assigned a role in the enzyme mechanism: it helps lock the substrate into place. The dynamics of this loop has been investigated by MD simulation. The initial coordinates were taken from the crystal structure of 2,3QD in the presence of the substrate kaempferol (KMP). After equilibration and simulation over 7.2 ns the substrate was removed and another equilibration and simulation of 7.2 ns was performed. The results show that the structures of the free enzyme as well as of the enzyme-substrate complex are stable in MD simulation. The linker shows strongly enhanced mobility in the loop region that is close to the entrance to the substrate cavity (residues 154-169). Movement of the loop takes place on a timescale of 5-10 ns. To confirm the conclusions about the loop dynamics drawn fromthe 7.2 ns simulation, the simulation was extended with another 8 ns. When substrate binds into the cavity the loop orders remarkably, although mobility is retained by residues 155-158. Some regions of the loop (residues 154-160 and 164-176) move over a considerable distance and approach the substrate closely, reinforcing the idea that they lock the substrate in the substrate cavity. The enthalpic component of the interaction of the loop with the protein and the KMP appears to favour the locking of the substrate. Two water molecules were found immobilised in the cavity, one of which exhibited rotation on the picosecond timescale. When the substrate is removed, the empty cavity fills up with water within 200 ps.

Ponte P. Bucci, Cubo 30C, 87030 Rende CS, Italy

Theoretical Chemistry and Biophysical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen,

Nijenborgh 4, 9747 AG Groningen, The Netherlands

Active site modeling in molecular dynamics simulations is investigated for the reduced state of copper azurin. Five simulation runs (5 ns each) were performed at room temperature to study the consequences of a mixed electrostatic/constrained modeling for the coordination between the metal and the polypeptide chain, using for the ligand residues a set of charges that is modified with respect to the apo form of the protein by the presence of the copper ion. The results show that the different charge values do not lead to relevant effect on the geometry of the active site of the protein, as long as bond distance constraints are used for all the five ligand atoms. The distance constraint on the O atom of Gly45 can be removed without altering the active site geometry. The coordination between Cu and the other axial ligand Met121 is outlined as being flexible. Differences are found between the bonds of the copper ion with the two apparently equivalent N(d1) atoms of His46 and His117.The overall findings are discussed in connection with the issue of determining a model for the active site of azurin suitable to be used in molecular dynamics simulations under unfolding conditions.

Department of Chemistry, Norwegian UniVersity of Science and Technology, 7491 Trondheim, Norway

Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen ø, Denmark

Utilizing a point-dipole interaction model, we present an investigation of the static polarizability and second hyperpolarizability of fullerenes and carbon nanotubes by varying their structure. The following effects are investigated: (1) the length dependence of the components of the static polarizability, (2) the static second hyperpolarizabilities of C_{60} and C_{70}, (3) the symmetry effects on the static second hyperpolarizability, (4) the length dependence of the components of the static second hyperpolarizability, and (5) the diameter dependence of the static second hyperpolarizability. It is demonstrated that the carbon nanotubes exhibit significantly larger second hyperpolarizabilities compared to a fullerene containing the same number of carbon atoms. Furthermore, the calculations show that the carbon nanotubes have a much larger directionality of the static second hyperpolarizability than the fullerenes.

Physical Chemistry and Molecular Thermodynamics, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands

Theoretical Chemistry, Materials Science Center, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Radiation Chemistry Department, Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands

This paper describes a quantum chemical study of the electronic structure of thienylene vinylene oligomers ranging in size from two thienylene rings (2TV) to 12TV. The geometries of the TV oligomers in the ground state, the lowest triplet state, and the singly and doubly oxidized states were optimized using density functional theory calculations. The electronic absorption spectra were obtained from configuration interaction calculations with an INDO/s reference wave function. Comparison with experimental data shows that the agreement is satisfactory, except for the triplet-triplet absorption spectra. For closed shell systems (ground state and doubly occupied state), the spectra were also calculated by time dependent density functional theory (TDDFT). TDDFT considerably underestimates the neutral singlet-singlet excitation energies for longer chains. The nature of the excited states for the TV radical cations was found to be more similar to that of thiophenes than to that of phenylene vinylenes, indicating that the sulfur atom has a marked influence on the π-electron system. For the (singlet) absorption spectra of doubly oxidized TVs, the results from TDDFT calculations are surprisingly good; they are also good for long chains. TDDFT calculations for doubly charged TVs also confirm the existence of a second, weak absorption band as has been found experimentally.

Department of Chemistry, Norwegian UniVersity of Science and Technology, 7491 Trondheim, Norway

Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen ø, Denmark

The dipole-dipole polarizability, α, and the second hyperpolarizability, γ, as well as the corresponding linear and third-order susceptibilities, χ^{(1)} and χ^{(3)}, have been calculated for C_{60} fullerene clusters by a point-dipole interaction (PDI) model. The size dependences of a linear chain, a mono-layer film, and a face-centered cubic crystal cluster have been investigated. It is found that the effects of the surrounding molecules on the molecular α and γ are large, in particular for the chain and the film because of the anisotropic surroundings, and that large clusters are required to obtain converged results. A localized PDI model gives the opportunity to divide α and γ into fragment contributions, and it is found that α and γ of molecules in the middle of the chain converge slower than the properties for the end molecules with respect to the length of the chain. Similar results are found for the mono-layer film. Finally, χ^{(1)} and χ^{(3)} have been calculated using a modified local-field theory including the induced dipole moments of the surrounding molecules explicitly. The corresponding refractive index and dielectric constant compare well with experiments. On the other hand, the comparison of χ^{(3)} with experiments is complicated by dispersion and vibrational contributions. Nonetheless, our value of χ^{(3)} is in good agreement with a recent quantum chemical calculation adopting a self-consistent reaction-field model.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Over the last couple of years, it has been shown that Time Dependent Density Functional Theory (TDDFT) is able to predict accurately and efficiently the polarizability of molecules, when using appropriate exchange-correlation potentials and (large) basis sets. In a previous paper, we compared the accuracy of the predicted mean polarizabilities of 15 organic molecules with experiment, and with two other computational methods: the Restricted Hartree-Fock (RHF) method and the Direct Reaction Field (DRF) approach, the first of which is ignored in this paper. The (empirical) DRF approach however was shown to give comparable accuracies to TD-DFT with the values computed in just a few seconds. In this paper, we use TD-DFT for computing the molecular polarizabilities of the twenty amino acid residues, and compare them with the results obtained with the DRF approach. Although the mean absolute deviation of the DRF values from the TD-DFT values is reasonable (7 %), it is more than two times the accuracy normally found with the DRF approach. Therefore we decided to optimize the atomic parameters for these systems, and found after optimization, a good agreement with the TDDFT values (mean absolute deviation 1.0 %). As the TD-DFT calculations were necessarily obtained with two additional hydrogens to saturate the backbone bonds, the molecular value of the polarizability of the amino acid residues is overestimated by the TD-DFT calculations. Therefore, the DRF approach (with the newly optimized atomic parameters) has been used to get the actual polarizabilities of the amino acid residues.

A new method for calculating the indirect nuclear spinspin coupling constant within the regular approximation to the exact relativistic Hamiltonian is presented. The method is completely analytic in the sense that it does not employ numeric integration for the evaluation of relativistic corrections to the molecular Hamiltonian. It can be applied at the level of conventional wave function theory or density functional theory. In the latter case, both pure and hybrid density functionals can be used for the calculation of the quasirelativistic spinspin coupling constants. The new method is used in connection with the infinite-order regular approximation with modified metric (IORAmm) to calculate the spin-spin coupling constants for molecules containing heavy elements. The importance of including exact exchange into the density functional calculations is demonstrated.

We study the π*←π singlet excitations of the π-conjugated oligomers of polyacetylene, polydiacetylene, polybutatriene, polythiophene, poly(para-phenylene vinylene) and the lowest singlet excitations of the hydrogen chain. For this we used time-dependent current-density functional theory within the Vignale-Kohn and adiabatic local density approximations. By studying the dependence of the excitation spectrum on the chain length we conclude that the reduction of the static polarizability when using the Vignale-Kohn functional has two origins. First, the excitation energies of transitions with a large transition dipole are shifted upward. Second, the HOMO-LUMO character and oscillator strength of the lowest transition within the adiabatic local density approximation is transferred to higher transitions. The lowest transitions that have a considerable oscillator strength obtained with the Vignale-Kohn functional have excitation energies that are in most cases in better agreement with available reference data then the adiabatic local density approximation.

The infinite-order regular approximation ~IORA! and IORA with modified metric (IORAmm) is used to develop an algorithm for calculating relativistically corrected isotropic hyperfine structure (HFS) constants. The new method is applied to the calculation of alkali atoms LiFr, coinage metal atoms Cu, Ag, and Au, the Hg^{+} radical ion, and the mercury containing radicals HgH, HgCH_{3}, HgCN, and HgF. By stepwise improvement of the level of theory from HartreeFock to second-order MllerPlesset theory and to quadratic configuration interaction theory with single and double excitations, isotropic HFS constants of high accuracy were obtained for atoms and for molecular radicals. The importance of relativistic corrections is demonstrated.

Department of Physics, Kaiserslautern University, Box 3049, D-67653, Kaiserslautern, Germany

Combining optical control theory with ab initio quantum chemistry and electronic crystal field theory we explore the laser induced femtosecond spin dynamics. We propose a scenario for ultrafast all-optical magnetic switching that results from the combination of spin-orbit coupling with appropriately shaped short laser pulses. We find that the application of the theory to the multiplet states within the gap of NiO(001) predicts for the first time the possibility of all-optical spin switching within 100 fs. The switching can be observed using any of the multiplets as the intermediate state.

We present a first-principles approach to the calculation of the electron-phonon interaction. This approachsolves some theoretical difficulties in the standard derivation of the electron-phonon interaction. We do notmake a Born-Oppenheimer approximation from the outset but transform the electronic coordinates to a frameattached to the nuclear framework. Subsequently coupled equations are derived which connect the nucleardensity-density correlation function to the electron Green function, the screened interaction, and the vertex.This set of equations is completely equivalent to the full problem and therefore higher-order effects aresystematically included. The derived equations are further compared to those obtained from the FröhlichHamiltonian. It is shown that careless use of this Hamiltonian leads to double counting but also insight is givenwhy use of this Hamiltonian has led to many useful results. Finally a simple method is presented that allowsfor the inclusion of electron-phonon coupling within a density-functional context.

It was recently proposed to use variational functionals based on many-body perturbation theory for the calculation of the total energies of many-electron systems. The accuracy of such functionals depends on the degree of sophistication of the underlying perturbation expansions. An older such functional and a recently constructed functional, both at the level of the GW approximation (GWA), were tested on the electron gas with indeed very encouraging results. In the present work we test the older of these functionals on atoms and find correlation energies much better than those of the random-phase approximation but still definitely worse ascompared to the case of the gas. Using the recent functional of two independent variables it becomes relatively easy to include second-order exchange effects not present in the GWA. In the atomic limit we find this to bevery important and the correlation energies improve to an accuracy of 10-20% when obtained from calculations much less demanding than those of, e.g., configuration-interaction expansions.

Departament de Quimica Fisica and Parc Cientific, Universitat de Barcelona, C. Marti i Franques 1, 08027 Barcelona, Spain

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The character of the electronic ground state of La0.5Ca0.5MnO3 has been addressed with quantum chemical calculations on large embedded clusters. We find a charge ordered state for the crystal structure reported by Radaelli et al. [Phys. Rev. B 55, 3015 (1997)] and Zener polaron formation in the crystal structure with equivalent Mn-sites proposed by Daoud-Aladine et al. [Phys. Rev. Lett. 89, 097205 (2002)]. Important O to Mn charge transfer effects are observed for the Zener polaron.

Theor Chem Acc (2003) 110: 34-41 Due to an unfortunate misunderstanding, the BLAP3 values reported in the original paper were not computed correctly; LAP3 correlation was not included in these calculations, so in fact the values reported as BLAP3 correspond to Becke exchange-only.

key concepts and exact functionals

We give an overview of the fundamental concepts of density functional theory. We give a careful discussion of the several density functionals and their differentiability properties. We show that for nondegenerate ground states we can calculate the necessary functional derivatives by means of linear response theory, but that there are some differentiability problems for degenerate ground states. These problems can be overcome by extending the domains of the functionals. We further show that for every interacting *v*-representable density we can find a noninteracting (i)v-representable density arbitrarily close and show that this is sufficient to set up a Kohn-Sham scheme. We finally describe two systematic approaches for the construction of density functionals.

In our previous work we developed the Finite Field method in order to calculate the fifth-order Raman response. The method was applied to calculate various polarization components of the two-dimensional response of liquid CS_{2}. So far, all calculations relied on the dipole-induced dipole. Accurate time-dependent density functional theory calculations have shown that this model has big discrepancies, when molecules are close together as in the liquid. We now report results of investigations on the importance of multipole and electron overlap effects on the polarizability and the fifth-order Raman response. It is shown that these collision effects, especially the induced multipoles, are crucial in the description of the fifth-order response. The impact is found to be especially pronounced for the *mmzzzz* response that is solely due to interaction induced effects. The calculated response will be compared with various experimental results.

Bonding, electric (hyper)polarizability and vibrational property of heterofullerene C_{48}B_{12} are studied by first-principles calculations. Infrared- and Raman-active vibrational frequencies of C_{48}B_{12} are assigned. In comparison to isolated carbon or boron atom, the static polarizability per atom in C_{48}B_{12} is enhanced due to the delocalized pi electrons. The first-order hyperpolarizability in C_{48}B_{12} is zero because of the inversion symmetry. The average second-order hyperpolarizability of C_{48}B_{12} is about 180% larger than that of C_{60}. Our results suggest that C_{48}B_{12} is an idea candidate for photonic and optical limiting applications because of the enhanced third-order optical nonlinearities.

For the quasi-relativistic normalized elimination of small component using an effective potential (NESC-EP) method, analytical energy gradients were developed, programmed, and implemented in a standard quantum chemical program package. NESC-EP with analytical gradients was applied to determine geometry, vibrational frequencies, and dissociation enthalpies of ferrocene, tungsten hexafluoride, and tungsten hexacarbonyle. Contrary to non-relativistic calculations and calculations carried out with RECPs for the same compounds, NESC-EP provided reliable molecular properties in good agreement with experiment. The computational power of NESC-EP results from the fact that reliable relativistic corrections are obtained at a cost level only slightly larger than that of a non-relativistic calculation.

All-electron CCSD(T), QCISD(T) and MP4(SDQ) calculations including relativistic effects via the use of the IORAmm Hamiltonian have been performed for PdCO. The optimized molecular geometry is in nice agreement with the recently obtained experimental data. The PdCO bond dissociation energy is estimated to be 38.8 kcal/mol. The vibrational spectrum of PdCO is calculated and a reassessment of the experimental datum for the frequency of bending mode is suggested.

This thesis is concerned with the investigation of the electronic structure of a number of insulating transition metal (TM) crystalline materials by using wave-function based embedded cluster calculations. Quantities and properties of interest studied in this work are related to the local ground-state electronic configuration, elementary, low-energy spin and charge excitations, and to core level excitation processes. Attempts to characterize and understand the electronic structure of solid TM compounds like oxides, halides, and silicides, began already in the 1940's. The main motivations at the time came from the issue of the Mott metal-insulator transition, the problem of magnetic ordering in insulators, and the problem of itinerant ferromagnetism. More recently, phenomena such as heavy fermion behavior, high-temperature superconductivity, colossal magnetoresistance, and spin-Peierls phase transitions have revived interest in these systems. Nevertheless, although considerable effort has been put into the field, many of the transition metal materials are poorly understood. The proper treatment of various competing physical effects, like electron localization as a result of strong electron-electron interactions and band-like behavior as a result of orbital overlap and translational symmetry, remains one of the most difficult problems in solid state physics. Within the quantum chemical approach the cluster method is directed to solving the Schrödinger equation for a small but relevant part of a larger system. Certain properties of crystalline solids, e. g. effects connected with the existence of isolated defects and impurities in an otherwise perfect infinite lattice, molecule-surface interactions, localized 3d or 4f electronic states in transition metal or rare earth compounds etc., are well suited to investigation by cluster methods. In the present work calculations were performed on clusters containing one or more TM sites plus the adjacent anions. The cluster is embedded in some effective potential that accounts for the crystal Madelung field and for short-range Pauli and exchange interactions due to the finite charge distribution of the nearest neighbors, respectively.

The third- and fifth-order time-domain Raman response can be calculated using a finite field method. This method will be described and compared to the time correlation function methods that can also be used. The advantages of the finite field method will be addressed and the calculated third- and fifth-order response will be presented for liquid carbon disulfide. The calculated third-order response is shown to agree very well with experiments. For the fifth-order response the problem of third-order cascaded response contaminating the experimental measurements will be addressed.

We give an overview of the underlying concepts of time-dependent current-densityfunctional theory (TDCDFT). We show how the basic equations of TDCDFT can be elegantly derived using the time contour method of nonequilibrium Green function theory. We further demonstrate how the formalism can be used to derive explicit equations for the exchange-correlation vector potentials and integral kernels for the Kohn-Sham equations and their linearized form.

In this work we demonstrate how to derive the Kohn-Sham equations of time-dependent current-density functional theory from a generating action functional defined on a Keldysh time contour. These Kohn-Sham equations contain an exchange-correlation contribution to the vector potential. For this quantity we derive an integral equation. We further derive an integral equation for its functional derivative, the exchange-correlation kernel, which plays an essential role in response theory. The exchange-only limits of the latter equation is studied in detail for the electron gas and future applications are discussed.

To describe the dynamical interplay of electronic and nuclear degrees of freedom in molecules exposed to strong laser pulses, we present two different variational approaches based on the statonary-action principle: A mean-field treatment of the electron-nuclear interaction and an explicitly correlated ansatz. The two methods are tested on a one-dimensional model of H_{2}^{+} which can be solved exactly. The correlated approach significantly improves upon the mean-field treatment, especially in the case of laser fields strong enough to cause substantial dissociation.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

A series of tri-nuclear metal clusters MS_{4}(M'PPh_{3})_{2}(M'PPh_{3}) (M=Mo,W; M' = Cu, Ag, Au) have been studied using density functional theory (DFT). The static polarizabilities and hyperpolarizabilities of the model clusters have been calculated using the finite-field (F-F) method. The model clusters, divided into two groups, are alike in that the structure of two fragments of rhombic units M-(m-S)2-M'(M=Mo,W; M'=Cu, Ag, Au), perpendicular to each other, are joined by sharing the node (bridge) metal M. It is the charge transfer from one of these moieties to the other in these characteristic sulfido-transitional metal cores that is responsible for the polarizabilities and hyperpolarizabilities. This kind of electronic de-localization, differentiated from that of planar p-system, is interesting and warrants further investigation. The structural effects on properties are important. In these models, considerable second-order nonlinearities are exhibited. The element substitution effect of Mo and W is weak, while that of Cu and Ag is relatively substantial. An overall order is, b_{xxxx}(Mo-Ag) > b_{xxxx}(W-Ag) > b_{xxxx}(Mo-Au) > b_{xxxx}(W-Au) > b_{xxxx}(Mo-Cu) > b_{xxxx}(W-Cu) and b_{av}(Mo-Ag) ~ b_{av}(W-Ag) > b_{av}(Mo-Au) ~ b_{av}(W-Au) ~ b_{av}(Mo-Cu) ~ b_{av}(W-Cu).

The division of a system under study in a quantummechanical and a classical system in QM/MM calculations is sometimes very natural, but a problem arises in the case of bonds crossing the QM/MM boundary. A new link model is presented that uses a capping (link) atom to satisfy the valences of the quantumchemical system, with the position of the capping atom depending on the positions of the real atoms involved in the link bond. This way no degrees of freedom for the capping atom are introduced. Moreover, the introduction of this artificial atom is corrected for by subtracting the classical molecular mechanics interactions with the real QM system it would have if it were a classical atom. That is, the capping atoms are Added and Removed. The new model has been tested on three amino acid residues, and shows a clear improvement over other link models (as represented by the IMOMM/ADF implementation). The average absolute deviation for the C_{a}-C_{b} bond distance as obtained when comparing the full QM and QM/MM results, is around 0.75 pm. The IMOMM model predicts distances for the C_{a}-C backbone and C_{a}-N backbone bonds with an average absolute deviation of 2.3-2.8 and 5.3-5.5 pm, respectively; this is an increase by a factor of 3.1-4.0 and 7.1-7.3 compared to the C_{a}-C_{b} bond. For the new AddRemove model, the average absolute deviations are 1.0-1.2 and 0.6-0.9 pm, respectively for the C_{a}-C backbone and C_{a}-N backbone bonds; compared to the C_{a}-C_{b} bond, this means only a slight change with a factor of 1.3-1.6 and 0.8-1.2 respectively. The new AddRemove model performs therefore much better, and is shown to be a substantial improvement over the IMOMM model.

1s ionization 1s->'4p' transition energies were determined by electronic structure calculations on embedded Mn ions and MnO_{6} clusters to identify mechanisms which determine the chemical shift observed in X-ray photoelectron and X-ray-absorption near-edge spectroscopy of manganese oxide compounds. The effective atomic charge, expressed by the 3d occupation, of the Mn-ion and the Madelung potential were shown to be the two most important influences on the observed shifts, by systemically varying these in the embedded cluster models. The relatively small sensitivity of the 1s ionization energy to the material is explained by the compensating effects of the Madelung potential and the effective atomic charge of the Mn-ion. The chemical shift in the 1s->'4p' transition energies in different materials is explained by the effects of Madelung potential and 3d occupation no longer compensating each other. This expresses the difference in the spatial extent of the 1s and 4p orbitals. Agreement with experimental shifts is only obtained upon including the screening effects by explicit treatment of the first layer of O-atoms around the Mn-ion.

We provide a successful approach towards the solution of the longstanding problem of the large overestimation of the static polarizability of conjugated oligomers obtained using the local density approximation within density-functional theory. The local approximation is unable to describe the highly nonlocal exchange and correlation effects found in these quasi-one-dimensional systems. Time-dependent current-density-functional theory enables us to describe ultranonlocal exchange-correlation effects within a local current description. Recently a brief account was given of the application of the Vignale-Kohn current-functional [G. Vignale, W. Kohn, Phys. Rev. Lett. 77 2037 (1996)] to the axial polarizability of oligomer chains [M. van Faassen, P.L. de Boeij, R. van Leeuwen, J.A. Berger, and J.G. Snijders, Phys. Rev. Lett. 88 186401 (2002)]. With the exception of the model hydrogen chain, our results were in excellent agreement with best available wavefunction methods. In the present work we further outline the underlying theory and describe how the Vignale-Kohn functional was implemented. We elaborate on earlier results and present new results for the oligomers of polyethylene, polysilane, polysilene, polymethineimine, and polybutatriene. The adiabatic local density approximation gave already good results for polyethylene, which were slightly modified by the Vignale-Kohn functional. In all other cases the Vignale-Kohn functional gave large improvements upon the adiabatic local density approximation. The Vignale-Kohn results were in agreement with best available data from wavefunction methods. We further analyze the hydrogen chain model for different bond length alternations. In all these cases the Vignale-Kohn correction upon the adiabatic local density approximation was too small. Arguments are given that further improvements of the functional are needed.

In this work we present theory and implementation for a discrete reaction field model within Density Functional Theory (DFT) for studying solvent effects on molecules. The model combines a quantum mechanical (QM) description of the solute and a classical description of the solvent molecules (MM). The solvent molecules are modeled by point charges representing the permanent electronic charge distribution, and distributed polarizabilities for describing the solvent polarization arising from many-body interactions. The QM/MM interactions are introduced into the Kohn-Sham equations, thereby allowing for the solute to be polarized by the solvent and vice versa. Here we present some initial results for water in aqueous solution. It is found that the inclusion of solvent polarization is essential for an accurate description of dipole and quadrupole moments in the liquid phase. We find a very good agreement between the liquid phase dipole and quadrupole moments obtained using LDA and results obtained with a similar model at the Coupled Cluster Singles and Doubles (CCSD) level of theory using the same water cluster structure. The influence of basis set and exchange correlation functional on the liquid phase properties was investigated and indicates that for an accurate description of the liquid phase properties using DFT a good description of the gas phase dipole moment and molecular polarizability are also needed.

We perform first-principles calculations of the structural, electronic, vibrational and magnetic properties of a novel C_{48}N_{12} aza-fullerene as well as C_{60}. Full geometrical optimization has shown that C_{48}N_{12} is characterized by several distinguish features: only one nitrogen atom per pentagon, two nitrogen atoms preferentially sitting together in one hexagon, S_{6} symmetry, six CN and nine CC bond lengths. The Mulliken analysis indicates that the doped nitrogen atoms in C_{48}N_{12} exist as electron acceptors and three-fourths of carbon atoms as electron donors. Total energy calculations of C_{48}N_{12} show that the highest occupied molecular orbital (HOMO) is a doubly degenerate level of a_{g} symmetry and the lowest unoccupied molecular orbital (LUMO) is a nondegenerate level with $a_{u}$ symmetry. The calculated binding and HOMO-LUMO energy gap of C_{48}N_{12} are about 1 eV smaller than those of C_{60}. For both C_{48}N_{12} and C_{60}, the total energies calculated with STO-3G, 3-21G and 6-31G basis sets differ from the 6-31G(d) basis set results by about 1.5%, 0.6% and 0.05%, respectively, and the HOMO-LUMO gap decreases about 5 eV and the binding energy increases about 2 eV due to electron correlations.

Vibrational frequency analysis predicts that C_{48}N_{12} has totally 116 vibrational modes: 58 modes are infrared-active (29 doubly-degenerate and 29 non-degenerate modes) and 58 modes are Raman-active (29 doubly-degenerate unpolarized and 29 non-degenerate polarized). For C_{48}N_{12}, the Raman-active frequency (RAF) of the strongest Raman singal in the low- and high-frequency regions and the lowest and highest RAFs are almost the same as those of C_{60}. It is found that C_{48}N_{12} exhibits 10 NMR (nuclear magnetic resonance) spectral signals. In comparison to isolated carbon or nitrogen atom, enhancement in the dipole polarizability is found due to the delocalized electrons in C_{48}N_{12} and C_{60}. Meanwhile, the effects of basis sets are discussed in detail. The different methods for calculating nuclear magnetic shielding tensors are compared.

Our detailed study of C_{60} reveals the importance of electron correlations and the choice of basis sets in the *ab initio* calculations. Our best calculated results for C_{60} with the B3LYP hybrid density functional theory are in excellent agreement with experiment and demonstrate the desirable efficiency and accuracy of this theory for obtaining quantitative information on the structural, electronic and vibrational properties of these materials. Our first-principles results suggest that C_{48}N_{12} could have potential applications as semiconductor components and possible building materials for nanometer electronics, photonic devices and good diamagnetic materials.

A Time Dependent Density Functional Theory Study

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

This paper discusses a time-dependent density functional theory (TDDFT) study of the effect of molecular structure on the excited state polarizability of conjugated molecules. A short phenylenevinylene oligomer containing three phenyl rings (PV2, distyryl benzene) is taken as a model system. Introduction of methyl side chain is shown to have only a small influence on the increase in polarizability upon excitation (the excess polarizability). Methoxy groups have a much larger effect but in this case the excess polarizability depends strongly on the dihedral angle between the side-chain and the backbone of the molecule. If the central phenyl ring of PV2 has a *meta*-configuration rather than *para*, both the optical absorption spectrum and the excess polarizability change considerably.

We present a Discrete Solvent Reaction Field (DRF) model for the calculation of frequency-dependent hyperpolarizabilities of molecules in solution. In this model the solute is described using Density functional Theory (DFT) and the discrete solvent molecules are described with a classical polarizable model. The first hyperpolarizability is obtained using Time-Dependent DFT in an efficient way by using the (2n+1) rule. The method was tested for liquid water represented as a water molecule embedded in a cluster of 127 classical water molecules. Frequency-dependent first and second hyperpolarizability related to the Electric Field Induced Second Hamonic Generation (EFISH) experiment was calculated both in the gas phase and in the liquid phase. For water in the gas phase, results in good agreement with correlated wavefunction methods and experiments are obtained by using the so-called shape-corrected exchange correlation (xc)-potentials. In the liquid phase the effect of using asymptotic correct functionals is discussed. Furthermore, it is shown that the first hyperpolarizability is more sensitive to damping of the interactions at short range than the second hyperpolarizability. The model reproduced the experimentally observed sign change in the first hyperpolarizaibility when going from the gas phase to the liquid phase.

A Discrete Solvent Reaction Field model for calculating frequency-dependent molecular linear response properties of molecule in solution is presented. The model combines a Time-Dependent Density Functional Theory (QM) description of the solute molecule with a classical (MM) description of the discrete solvent molecules. The classical solvent molecules are represented using distributed atomic charges and atomic polarizabilities. All the atomic parameters have been chosen so as to describe molecular gas phase properties of the solvent molecule, i.e. the atomic charges reproduce the molecular dipole moment and the atomic polarizabilities resproduce the molecular polarizability tensor using a modified dipole interaction model. The QM/MM interactions are introduced into the Kohn-Sham equations and all interactions are solved self-consistent, thereby allowing for the solute to be polarized by the solvent. Furthermore, the inclusion of polarizabilities in the MM part allows for the solvent molecules to be polarized by the solute and by interactions with other solvent molecules. Initial applications of the model to calculate the vertical electronic excitation energies and frequency-dependent molecular polarizability of a water molecule in a cluster of 127 classical water molecules are presented. The effect of using different exchange correlation (xc)-potentials is investigated and the results are compared with results from wavefunction methods combined with a similar solvent model both at the correlated and uncorrelated level of theory. It is shown that accurate results in agreement with correlated wavefunction results can be obtained using xc-potentials with the correct asymptotic behavior.

Department of Chemistry, H. C. Ørsted Institute, University of Copenhagen, DK-2100~Copenhagen Ø, Denmark

A dipole interaction model (IM) for calculating the molecular second hyperpolarizability, Gamma, of aliphatic and aromatic molecules has been investigated. The model has been parametrized from quantum chemical calculations of Gamma at the self-consistent field (SCF) level of theory for 72 molecules. The model consists of three parameters for each element *p*: an atomic polarizability, an atomic second hyperpolarizability, and an atomic parameter, Phi_{p}, describing the width of the atomic charge distribution. The Phi_{p} parameters are used for modeling the damping of the interatomic interactions.

Parameters for elements H, C, N, O, F and Cl were determined and typical differences between the molecular Gamma derived from quantum chemical calculations and from the IM are below 30% and on average around 10%.

As a preliminary test, the dipole interaction model was applied to the following molecular systems not included in the training set: the urea molecule, linear chains of urea molecules, and C_{60}. For these molecules deviations of the IM result for the molecular Gamma from the corresponding SCF value were at most around 30% for the individual components, which in all cases is a better performance than obtained with semi-empirical methods.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The geometries of various tautomers and isomers of 2-methylamino-2-imidazoline, 2-methylamino-2-oxazoline, 2-methylamino-2-thiazoline, 2-phenylamino-2-imidazoline, 2-phenylamino-2-oxazoline and 2-phenylamino-2-thiazoline have been studied using the Becke3LYP/6-31+G(d, p) DFT, ONIOM(Becke3LYP/6-31+G(d, p):HF/3-21G*) and ONIOM(Becke3LYP/6-31+G(d, p):AM1) methods. The optimised geometries indicate that these molecules show a distinctly nonplanar configuration of the cyclic moieties. In the gas phase the amino tautomers (with exception of 2-phenylamino-2-imidazoline) are computed to be more stable than the imino tautomers. Of the two possible (E and Z) isomers of methyl and phenyl derivatives of imino-oxazolidine and imino-thiooxazolidine species the (Z)-isomers have the lowest energy. The iminozation free energies in the gas phase were found to be 5 - 15 kJ/mole. Absolute values of KT depend strongly on the accuracy of the method used for calculation of free energy. Solvation (using the MD simulations) causes in most cases a shift in tautomeric preference towards the imino species.

Analytic expressions are derived for the evaluation of derivatives of the total molecular energy with respect to external parameters ~nuclear coordinates, external electric fields, etc.! within the relativistic regular approximation. The presented formalism employs the spectral resolution of the identity avoiding, however, the explicit use of an auxiliary basis set in the calculation of the matrix elements of the regular relativistic Hamiltonian. The final formulas for the total energy and energy derivatives are presented in matrix form suitable for implementation into standard quantum chemical packages. Results of benchmark calculations for gold containing diatomic molecules and for xenone hexafluoride performed at the HartreeFock and various correlation corrected levels of theory are presented and discussed.

A new method for relativistically corrected nuclear magnetic resonance (NMR) chemical shifts is developed by combining the individual gauge for the localized orbital approach for density functional theory with the normalized elimination of a small component using an effective potential. The new method is used for the calculation of the NMR chemical shifts of ^{95}Mo and ^{183}W in various molybdenum and tungsten compounds. It is shown that quasirelativistic corrections lead to an average improvement of calculated NMR chemical shift values by 300 and 120 ppm in the case of ^{95}Mo and ^{183}W, respectively, which is mainly due to improvements in the paramagnetic contributions. The relationship between electronic structure of a molecule and the relativistic paramagnetic corrections is discussed. Relativistic effects for the diamagnetic part of the magnetic shielding caused by a relativistic contraction of the *s,p* orbitals in the core region concern only the shielding values, however, have little consequence for the shift values because of the large independence from electronic structure and a cancellation of these effects in the shift values. It is shown that the relativistic corrections can be improved by level shift operators and a B3LYP hybrid functional, for which Hartree-Fock exchange is reduced to 15%.

The exact relativistic Hamiltonian for electronic states is expanded in terms of energy-independent linear operators within the regular approximation. An effective relativistic Hamiltonian has been obtained, which yields in lowest order directly the infinite-order regular approximation (IORA) rather than the zeroth-order regular approximation method. Further perturbational expansion of the exact relativistic electronic energy utilizing the effective Hamiltonian leads to new methods based on ordinary (IORAn) or double [IORAn(2)] perturbation theory (n: order of expansion), which provide improved energies in atomic calculations. Energies calculated with IORA4 and IORA3(2) are accurate up to c^{20}. Furthermore, IORA is improved by using the IORA wave function to calculate the Rayleigh quotient, which, if minimized, leads to the exact relativistic energy. The outstanding performance of this new IORA method coined scaled IORA is documented in atomic and molecular calculations.

Analytic expressions for the derivatives of the total molecular energy with respect to external electric field are derived within the regular approximation to the full four-component relativistic Hamiltonian and presented in matrix form suitable for implementation in standard quantum-chemical codes. Results of benchmark calculations using the *infinite-order regular approximation with modified metric* method are presented and discussed. The static electric dipole polarizabilities of group VIII metal tetroxides MO_{4} for M=Ru, Os, Hs (Z=108) are studied with the help of second-order Møller-Plesset perturbation theory using the infinite-order regular approximation with modified metric Hamiltonian. The polarizabilities obtained vary in the sequence RuO_{4}>OsO_{4}>HsO_{4}, which is different from those obtained in other studies. However, it is in line with calculated ^{1}T_{2}^{←1}A_{1} excitation energies of the group VIII tetroxides, which provide a measure for the magnitude of their polarizabilities.

Early theoretical studies of magnetic interactions between paramagnetic centers in solids and molecules are briefly reviewed as an introduction to the main theme of this paper: non-orthogonal CI approaches for the prediction and interpretation of magnetic couplings. In a non-orthogonal CI approach, the wavefunctions are linear combinations of configuration state functions, which are each expressed in their own optimized orbital basis set. The NOCI approach allows for an adequate treatment of near-degeneracy correlation effects using a compact, transparent wavefunction. This facilitates straightforward analysis of the physical effects involved. A closely related method is State Interaction, where the final wavefunctions are linear combinations of multi-configuration functions. Comparisons are made with the use of conventional configuration interaction and perturbation theory methods. The compound La_{2}CuO_{4} is selected as an illustrative example.

By expanding the Foldy-Wouthuysen representation of the Dirac equation near the free-particle solution it is shown that the Hamiltonian of the zeroth-order regular approximation (ZORA) leads to an infinite summation of the leading relativistic corrections to the free-particle, non-relativistic energy. The analysis of the perturbation expansion of the ZORA Hamiltonian reveals that the ZORA Hamiltonian recovers all terms of the Breit-Pauli theory to second order. This result is general and applies not only to hydrogen-like atomic ions (as was demonstrated before) but also to a wide variety of physical problems. ZORA is analogous to the random phase approximation in many-body theory in the sense that both methods include an infinite-order summation of the asymptotically non-vanishing terms. This highlights the difference between ZORA and the Douglas-Kroll method, with the latter being analogous to finite-order many-body perturbation theory. On the basis of this analysis the performance of ZORA when calculating various molecular properties is discussed.

Department of Chemistry, Norwegian University of Science and Technology,N-7491 Trondheim, Norway

Department of Chemistry, H. C. Ørsted Institute, University of Copenhagen, DK-2100~Copenhagen Ø, Denmark

An atomic dipole interaction model has been used for calculating the second hyperpolarizability of carbon nanotubes on the length scale up to 75~nm. It is demonstrated that an atomistic representation of mesoscale systems such as nanotubes can be used to obtain a cubic response property up to a size of the system where the property scales linearly with increasing size. In particular, it demonstrates that atomistic models are useful also for designing nonlinear molecular materials, where local modifications may give large macroscopic contributions. The saturation length has been calculated for carbon nanotubes. It is found that carbon nanotubes are comparable to conjugated polymers with respect to the magnitude of the second hyperpolarizability and are therfore very promising candidates for future nonlinear optical materials.

Radon hexafluoride is a bound species (bond length Rn-F: 2.008 Å) as demonstrated by correlation corrected relativistic ab initio calculations using IORAmm (infinite order regular approximation with modified metric) at the MP2 level of theory with a (24s20p13d8f)[15s13p8d4f]/aug-cc-pVDZ basis set. The calculated atomization energy is 226.9 kcal mol^{-1} and the dissociation energy leading to Rn and 3F_{2} is 126.6 kcal mol^{-1}. Results are in line with simple orbital-based predictions of possible relativistic effects. The relativistic effect for the atomization energy is 10.8 kcal mol^{-1} rather than +27.7 kcal mol^{-1} as predicted on the basis of Dirac-Hartree-Fock (DHF) calculations. The latter were flawed by the lack of correlation corrections and an erroneous polynomial fit of the potential energy surface in the vicinity of the global minimum.

An embedded cluster approach was applied to study the electronic excitations on the NiO(001) surface. Using a quantum chemical calculation, a small (NiO5)8- cluster was embedded in a set of point charges to model the NiO(001) surface. Starting from the unrestricted Hartree-Fock level of theory, we calculate the ground-state properties to provide some insight into electronic structure and excitation. We estimate the excitation energies and oscillator strengths using the single excitation configuration-interaction (CIS) technique. Our results show that the CIS method is reasonably accurate for estimating the low-lying d-d excitations below the gap. We then demonstrate the electron correlation effects on the d-d transitions at several levels of ab initio correlated theory [CID (with all double substitutions), CISD (with all single and double substitutions), (quadratic) QCISD, and (with all single, double, and triple substitutions) QCISD (T)]. The electron correlation tends to decrease the magnitude of d-electron excitation energies. Using the many-body wave functions and energies resulting from CID and QCISD(T) calculations, we compute the second harmonic generation (SHG) tensor for the NiO(001) surface. In contrast to bulk NiO, where the SHG response is forbidden within the electric-dipole approximation because of the inversion symmetry, the C4v symmetry of the surface leads to five nonzero tensor elements. From that, the intensity of the nonlinear optical response as a function of photon energy at different polarizations of the incident and outgoing photons is obtained. This quantity can be directly measured in experiment, and we suggest possible conditions in order to detect it.

Nijenborgh 4, 9747 AG Groningen, The Netherlands

Department of Physical and Inorganic Chemistry, Universitat Rovira i Virgili,

Placa Imperial Tarraco 1, Tarragona 43005, Spain

Results of *ab initio* embedded cluster calculations indicate that the doublet ground state of the V-O* _{R}*-V rung originates from a V 3

The fifth-order 2D Raman response of a liquid is calculated taking all possible interaction induced effects into account. Next to dipole-induced dipole interactions, close collision effects due to induced multipoles and electron overlap are found to give a significant contribution to the response of liquid carbon disulfide. A correct prediction of the spectrum is impossible, when these effects are not properly taken into account. The calculated response is found to be in good agreement with some of the most recent experiments.

The geometries of a set of small molecules were optimized using eight different exchangecorrelation potential in a few different basis sets of Slater type orbitals, ranging from a minimal basis (I) to a triple zeta valence basis plus double polarization functions (VI). This enables a comparison of the accuracy of the xc-potentials in a certain basis set, which can be related to the accuracies of wavefunction based methods like Hartree-Fock and Coupled Cluster. Four different checks are done on the accuracy by looking at the mean error, standard deviation, mean absolute error and maximum error. It is shown that the mean absolute error decreases with increasing basis set size, and reaches a basis set limit at basis VI. With this basis set, the mean absolute errors of the xc-potentials are of the order of 0.7-1.3 pm, which i s comparable to the accuracy obtained with CCSD and MP2/MP3 methods. In the second part of this paper, the geometry of five metallocenes is optimized with the same potentials and basis sets, either in a non-relativistic or a scalar relativistic calculation using the ZORA approach. For the first row transition metal complexes, the relativistic corrections have a negligible effect on the optimized structures, but for ruthenocene they improve the optimized Ru-ring distance by some 1.4-2.2 pm. In the largest basis set used, the absolute mean error is again of the order of 1.0 pm. As the wavefunction based methods either give a poor performance for metallocenes (Hartree-Fock, MP2), or the size of the system makes a treatment with accurate methods like CCSD(T) in a reasonable basis set cumbersome, the good performance of Density Functional Theory calculations for these molecules is very promising. Even more so as DFT is an efficient method that can be used without problems on system sizes of this kind, or larger.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

A series of large molecular quadratic hyperpolarizabilities in donor/acceptor substituted *trans*-tetraammineruthenium(II) compelexes [Ru(NH_{3})_{4}L^{D}L^{A}]^{n+} (*n* = 2, L^{D} = 4-(dimethylamino)pyridine, L^{A} = 4-pyridinecarboxaldehyde (**1**), 4-acetylpyridine (**2**), ethyl isonicotinate (**3**), or *n* = 3, L^{A} = *N*-methyl-4,4'-bipyridinium (**4**), *N*-(4-acetylphenyl)-4,4'-bipyridimium (4-AcPhQ^{+}) (**5**), and *n* = 3, L^{D} = NH_{3}, L^{A} = (4-AcPhQ^{+}) (**6**) has been studied using the TDDFT and *ab initio* HF method. It is found that the magnitude of the static hyperpolarizabilities b_{0} increases as the donor/acceptor strength of L^{D}/L^{A} increases. The co-planes of the pyridine or benzene ring are not necessary to maintain the large non-linear optical properties. According to our study on Ru complexes, the TDDFT method is more reliable than the HF method in b-calculations.

Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC, Canada V6T 1Z1

In this work we have investigated the effects of substituting carbon atoms with B and N on the 2nd hyperpolarizability of C_{60} using time-dependent density functional theory. We have calculated the 2nd hyperpolarizability of the double substitute-doped fullerenes C_{58}NN, C_{58}BB and C_{58}BN. For C_{60} only small changes in the 2nd hyperpolarizability were found when doping with either 2 B or 2 N. However, by doping C_{60} with both B and N, creating an donor-acceptor system, an increase in the 2nd hyperpolarizability with about 50% was found.

A recently developed variationally stable quasi-relativistic method, which is based on the low-order approximation to the method of normalized elimination of the small component, was incorporated into density functional theory (DFT). The new method was tested for diatomic molecules involving Ag, Cd, Au, and Hg by calculating equilibrium bond lengths, vibrational frequencies, and dissociation energies. The method is easy to implement into standard quantum chemical programs and leads to accurate results for the benchmark systems studied.

With the help of resolution of the identity (RI) a compact representation for the zeroth-order (ZORA) and infiniteorder (IORA) regular approximation Hamiltonians in matrix form is developed. The new representation does not require calculation of any additional molecular integrals, which involve an auxiliary basis set used in the RI. The IORA computational scheme is modified in such a way that the erroneous gauge dependence of the total energy is reduced by an order of magnitude. The new quasi-relativistic method, dubbed IORAmm, is tested along with the ZORA and IORA methods in atomic and molecular calculations performed at the SCF and MP2 level.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The static polarizabilities and the second-order hyperpolarizabilities of a series of tri-nuclear metal cluster models MS_{4}(M'PPh_{3})_{2}(M'PPh_{3}) (M=Mo,W; M'=Cu, Ag, Au) have been calculated within the first-principle theoretical framework. The model clusters have two fragments of rhombic units and it is the charge transfer from one of these moieties to the other that is responsible for nonlinear optical property. This kind of electronic delocalization, differentiated from that of planar pi-system, is very interesting and is worthy for further investigation.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The objective of the work presented in this chapter has been to make a contribution to the development of quantum biology by carrying out the first density functional theoretical (DFT) investigation on larger segments of deoxyribonucleic acid (DNA). The challenges associated with this objective are twofold. In the first place, we wish to describe the structure and energetics of the DNA segments accurately and, in particular, we try to achieve a better understanding of the nature and behavior of this complex molecule of heredity on the basis of its electronic structure. Our analyses highlight the covalent character of hydrogen bonds in Watson-Crick pairs. Furthermore, they lead to the solution of a hitherto unresolved discrepancy between experimental (X-ray) and theoretical (ab initio and DFT) structures of AT (or AU) and GC base pairs. In the second place, the computational effort connected with first-principles quantum chemical studies on these biochemical systems is enormeous and, until recently, calculations on these systems have been out of reach. Thus, finding and implementing speed-up techniques that make our model systems computationally accessible constitutes the other challenge of this work that, in fact, had to be tackled first.

No abstract available

No abstract available

Department of Organic Chemistry and The Lise Meitner-Minerva Center for Computational Quantum Chemistry, Hebrew University, Jerusalem, 91904 Jerusalem, Israel

Multi-reference effects can be covered by density functional theory (DFT) either implicitly via the exchange-correlation functional or explicitly via the form of the Kohn-Sham wave function. With the help of the exchange hole it is shown that the self-interaction error of the exchange functional will mimic long-range electron correlation effects if restricted Kohn-Sham theory is used. Functionals based on Slater or Becke exchange have a relatively large self-interaction error and, therefore, lead to a relatively large implicit coverage of long-range correlation, which, because of the possibility of doublecounting of electron correlation, has to be considered when using these functionals in connection with two- or multi-configurational descriptions based on ensemble DFT methods such as REKS (spin- Restricted Ensemble-referenced KS-DFT). Arguments are given that a REKS description of a multireference problem avoids a double-counting of long-range correlation effects, in particular as in this situation the self-interaction error of the exchange functional simulates more short- rather than longrange correlation effects. There is, however, no guarantee that the short-range effects are not doublecounted, namely once via the exchange and once via the correlation functional. Therefore, one should use hybrid functionals such as B3LYP in connection with multi-reference DFT methods because for hybrid functionals the self-interaction error and by this the implicit coverage of long(short)-range correlation effects is reduced due to the admixture of exact exchange. This rule applies also to broken-symmetry UDFT, which performs better with hybrid rather than GGA functionals. A way of avoiding the implicit coverage of multi-reference effects is given by the combination of wave function theory and DFT methods. The advantages and disadvantages of CAS-DFT are discussed and it is shown that an effective reduction of a double-counting of correlation effects is possible within this method.

Effect of Torsional Disorder and Polymerization Defects

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The pulse-radiolysis time-resolved microwave conductivity technique was used to measure the mobility of charges along isolated chains of conjugated polymers. The mobility of holes along poly(phenylenevinylene) and polythiophene backbones were measured to be 0.43 cm^{2}V^{-1}s^{-1} and 0.02 cm^{2}V^{-1}s^{-1}, respectively. The large difference between the mobility of holes on poly(phenylenevinylene) and polythiophene chains can be attributed to deviations from the coplanar alignment of structural units in the polymer backbone. The effect of such torsional disorder on intramolecular hole transport was theoretically investigated using a model based on the tight-binding approximation. The calculated ratio of hole mobilities along poly(phenylenevinylene) and polythiophene chains was found to be in agreement with experimental findings. For both polymers, estimated mobilities become consistent with the experimental values if polymerization defects and chain end effects are included in the calculations. This suggests that even higher mobilities than those reported here can be realized by improving the effective conjugation along the polymer chain.

A model of the polarizability of carbon disulfide dimers was constructed, using polarizabilities from accurate time-dependent density functional theory calculations as reference. This direct reaction field model takes dipole-induced dipole effects, induced multipole effects and effects due to the overlap of the electronic clouds into account in an approximate way. The importance of the induced multipole and the overlap effects is investigated. This polarizability model is subsequently used to calculate the third-order time-domain Raman response of liquid carbon disulfide. These results are compared to experimental data and earlier calculated response in which only dipole-induced dipole effects on the polarizability were included. The multipole effects are found to give a significant contribution to the subpico second part of the third-order Raman response.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Nonlocal interactions play a prominent role in the optics of inhomogeneous systems. Traditional discrete dipole descriptions take into account only electromagnetic nonlocality. This is insufficient to describe correctly the inhomogeneous optical response (reflectance anisotropy e.g.) for strongly bonded systems like semiconductor surfaces. For those systems exists also a prominent quantum mechanical nonlocality. In a cellular descripton this can be understood easily from the behavior of the wave function. For strongly bonded systems the wave function extends across cell boundaries and cells can only be polarized, when neigboring cells get polarized as well. This quantum induction introduces nonlocal polarizabilities in the description. The technical details how discrete dipole models have to be adapted to use nonlocal polarizabilities in finite systems and crystalline slabs and surfaces are given in this paper. The modified method is called discrete cellular method.

Materials Research Department, Risø National Laboratory, DK-4000 Roskilde, Denmark

Department of Chemistry, H. C. Ørsted Institute, University of Copenhagen, DK-2100~Copenhagen Ø, Denmark

We have developed and investigated a dipole interaction model for calculating the polari zability of molecular clusters. The model has been parametrized from the frequency-dependent molecular polarizability as obtained from quantum chemical calculations for a series of 184 aliphatic, aromatic and hetero-cyclic molecules. A damping of the interatomic interaction at short distances is introduced in such a way as to retain a traceless interaction tensor and a good description of the damping over a wide range of interatomic distances. By adopting atomic polarizabilities in addition to atom-type parameters describing the damping and the frequency-dependence, respectively, the model is found to reproduce the molecular frequency-dependent polarizability tensor calculated with *ab initio* methods. A study of the polarizability of four dimers has been carried out: the hydrogen fluoride, methane, benzene and urea dimers. We find in general good agreement between the model and the quantum chemical results over a wide range of intermolecular distances. To demonstrate the power of the model, the polarizability has been calculated for a linear chain of urea molecules with up to 300 molecules and one- and two-dimensional clusters of C_{60} with up to 25 molecules. Substantial intermolecular contributions are found for the polarizability anisotropy, whereas the effects are small on the mean polarizability. For the mean polarizability of C_{60}, we find good agreement between the model and experiments both in the case of an isolated molecule and in a comparison of a planar cluster of 25 C_{60} molecules with experimental results on thin films.

The subpicosecond dynamics of binary mixtures of carbon disulfide and alkane have been studied using third-order time-resolved Raman techniques. Both the anisotropic and the isotropic responses were investigated. These depend differently on many-body contributions to the first-order susceptibility and probe different modes in the liquid. The anisotropic response is dominated by single molecule effects, whereas the isotropic response is completely determined by many-body contributions since the single molecule response vanishes. To interpret the experimental results, molecular dynamics simulations were performed on model mixtures. The effect of dilution on the subpicosecond response also cannot be explained by many-body effects in the first-order susceptibility alone. Explanations such as aggregation due to quadrupole moments on the carbon disulfide molecules and density changes cannot explain the observed dilution effects. Apparently the character of the many-body dynamics itself is modified by the change of the molecular force fields, when carbon disulfide molecules are replaced by alkanes.

Theoretical Chemistry and Polymer Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

In this paper a combined experimental and quantum chemical study of the geometry and opto-electronic properties of unsubstituted and dialkoxy-sustituted phenylene-vinylene oligomers (PV's) is presented. The optical absorption spectra for PV cations with different chain lengths and substitution patterns were measured using pulse radiolysis with time-resolved spectrophotometric detection from 1380 to 500 nm (0.9 to 2.5 eV). The geometries of the PV's studied were optimized using density functional theory (DFT) for both the neutral and singly charged molecule. The spectra for the PV radical cations were then calculated using singly excited configuration interaction with an intermediate neglect of differential overlap reference wave function method together with the DFT geometry. The agreement between experimental and theoretical absorption energies is excellent; most of the calculated radical cation absorption energies are within 0.15 eV of the experimental values. The pattern of dialkoxy-substitution is found to have a large effect on the optical absorption spectrum of the cation. Using the calculated charge distribution it is shown that the degree of delocalization of the charge correlates with the energy of the lowest absorption band. If alkoxy side chains are present on some of the rings the positive charge tends to localize at those sites.

The collision induced effects in the third-order Raman response of liquid xenon have been studied theoretically and experimentally. The effect of electron cloud overlap on the polarizability of xenon dimers have been studied using accurate time-dependent density functional theory calculations. The dimer polarizabilities have been modeled using a direct reaction field model that can be generalized to condensed systems. The polarizability model has been used in molecular dynamics simulations to calculate the third-order time-domain Raman response of liquid xenon. Excellent agreement is found between the shape of the calculated and the measured response. The shape of the calculated response depends little on whether the electron overlap effect is taken into account, but the intensity of the response is strongly affected by the electron overlap effect.

We have studied the medium effects on the frequency-dependent polarizability of water by separating the total polarizability of water clusters into polarizabilities of the individually water molecules. A classical frequency-dependent dipole-dipole interaction model based on classical electrostatic and an Unsöld dispersion formula has been used. It is shown that the model reproduces the polarizabilities of small water clusters calculated with time-dependent density functional theory. A comparison between supermolecular calculations and the localized interaction model illustrates the problems arising from using supermolecular calculations to predict the medium perturbation on the solute polarizability. It is also noted that the solute polarizability is more dependent on the local geometry of the cluster than on the size of the cluster.

A Direct Reaction Field formulation.

Radiation Chemistry Department, Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands

A consistent derivation is given for local field factors to be used for correcting measured or calculated static (hyper-)polarizabilities in the condensed phases. We show how local fields should be used in the Coupled Perturbative Hartree Fock (CPHF) or Finite Field methods for calculating these properties, specifically for the Direct Reaction Field approach, in which a quantum chemically treated 'solute' is embedded in a classical 'solvent' mainly containing discrete molecules. The derivation of the local fields is based on strictly linear response of the classical parts and they are independent of any quantum mechanical method to be used.

Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR. China

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Theoretical studies and simulation have been applied to explore novel nonlinear optical crystals in metal clusters. The structure-nonlinear optical property relationships of a series of the metal cluster molecules have been investigated theoretically within the density functional theory (DFT) framework. For example, the polarizability and hyperpolarizability of a set of three-nuclear metal cluster compounds of Mo(W)/Cu(Ag, Au) sulfur system are calculated to elucidate the influence of geometric configuration and the element substitution effect; a set of potential second harmonic generation (SHG) metal cluster crystals are studied and simulated such as, MoAg_{2}S_{5}(Py)(PPh_{3})_{2}, MoS_{4}Cu_{4}I_{2}(Py)_{6} cluster and more. The results indicate many of these crystals are promising SHG crystals that may be applied in infrared (IR) spectroscopic region. The studies are useful to the procedure of screening, simulations and design of novel nonlinear optical crystals in metal cluster compounds, especially those to be applied in medium/far-IR region.

Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The switching behaviour of 1,2-bis(5-phenyl-2-methyl-thien-3-yl)cyclopentene is studied by means of polarization selective nonlinear optical spectroscopy and time-dependent density functional theory. The combined information from the observed population and orientational dynamics together with the results of theoretical calculations show that on a subpicosecond time scale rapid mixing and relaxation of electronic states occur, before switching takes place. Such pre-switching dynamics was not studied in detail in these systems before. Then, the switching process itself occurs by the formation of a C-C bond in the central cyclopentane ring with a time constant of 4.2 ps. Driven by the ring closure, the side groups of the switch molecules rotate to a nearly coplanar conformation with a time constant of about 8 ps. The switching process is completed by relaxation of the vibrationally hot ground state of the closed form of the molecule to thermal equilibrium.

Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR. China

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The oriented-gas model based on additivity hypothesis is widely used in predicting macroscopic the nonlinear optical susceptibility of a molecular crystal from molecular hyperpolarizability calculations. Here, we argue that the intermolecular hydrogen bond interactions will break the additivity relationship for the first hyperpolarizability of urea hydrogen-bonded clusters up to the nearest-neighbor configuration on the basis of our ab initio and time-dependent density functional theory (TDDFT) studies. The influences of basis sets, exchange-correlation potential and frequency dispersion on TDDFT calculation of (hyper)polarizability are discussed as well. We hope that the study will be helpful to the molecular design and simulations of novel nonlinear optical materials.

Department of Chemistry, Texas A&M University, USA

*Ab initio* theoretical results are reported to determine the role of inter-atomic screening of the metal core hole in Mn 3s X-ray photoelectron spectra, XPS, of MnO. We focus on the transitions to high spin 3s core-hole states. We have used configuration interaction wavefunctions within the framework of non-orthogonal orbitals for different configurations. This method allows for a balanced treatment of configurations that involve different degrees of screening of the core-hole. The differences between MnO and NiO are analyzed. In MnO inter-atomic screening of the core hole is found to play a minor role. This is in contrast with NiO, where, in previous work, the inclusion of inter-atomic screening of the metal core hole was shown to be crucial for a proper explanation of the Ni 3s XPS. The main reason for the difference is an essentially atomic effect, namely the larger electron affinity of Mn as compared to Ni. This difference is only partly compensated by the smaller crystal field in MnO.

We solve the longstanding problem of the large overestimation of the static polarizability of conjugated polymers obtained using the local density appro ximation within density functional theory. The local approximation is unable to describe the highly nonlocal exchange and correlation effects found in these quasi-onedimensional systems. Time-dependent current density functional theory enables us to describe ultra nonlocal exchange-correlation effects within a semi-local current description. For this we use the Vignale-Kohn functional [G. Vignale, W. Kohn, Phys. Rev. Lett. 77 2037 (1996)] and obtain the static polarizability of several polymers. The results are in excellent agreement with best available correlated wavefunction methods.

Department of Physical and Inorganic Chemistry, Universitat Rovira i Virgili, Placa Imperial Tarraco 1, Tarragona 43005, Spain

We provide evidence for a doublet ground state of the V-O-V rung of predominant V 3*d*_{xy}^{1}-O 2*p*_{y}^{1}-V 3*d*_{xy}^{1} character in the HT phase of a-NaV_{2}O_{5}. By *ab initio* quantum chemical embedded cluster calculations, it is shown that such a model is able to explain the main features of the optical absorption spectrum, and the AFinteraction along the *b* axis. The unpaired electron on O is low-spincoupled to the V *d* electrons and spin density is predicted to be localized on vanadium. The optical absorption peak at 0.9 eV is assigned to a state with similar orbital occupations but a different spin coupling scheme, resulting in spin density localized on the oxygen of the V-O-V rung. Absorption peaks at higher energy are tentatively assigned to vanadium to rung oxygen and apex oxygen to vanadium charge transfer excitations. Our analysis suggests that the high temperature magnetic structure can still be described by a spin model with an effective S=1/2 spin on each V-O-V rung.

Photon Absorption and the Isomerization of the Chromophore

The Biocenter and the Department of Biochemistry, the University of Oulu, Faculty of Sciences, P.O. Box 3000, FIN-90401 Oulu, Finland

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Molecular Dynamics simulation techniques together with Time-Dependent Density Functional Theory calculations have been used to investigate the effect of photon absorption by a 4-hydroxy-cinnamic acid chromophore on the structural properties of the Photoactive Yellow Protein (PYP) from Ectothiorodospira halophila. In this bacteria exposure to blue light leads to a negative phototactic response. The calculations suggest that the protein not only modifies the absorption spectrum of the chromophore, but also regulates the subsequent isomerization of the chromophore by stabilizing the isomerization transition state. Although signalling from PYP is thought to involve partial unfolding of the protein, the mechanical effects accompanying isomerization do not appear to directly destabilize the protein.

In this article we review time-dependent density functional theory for calculating the static and frequency-dependent dielectric function e(w) of nonmetallic crystals. We show that a real-space description becomes feasible for solids by using a combination of a lattice-periodic (microscopic) scalar potential with a uniform (macroscopic) electric field for the description of the effective one-electron system. We treat the time-dependent fields as perturbations in a periodic structure calculation. The induced density and microscopic potential can be obtained self-consistently for fixed macroscopic field by using linear response theory in which Coulomb interactions and exchange-correlation effects are included. The dielectric function can then be obtained from the induced current. We obtained e(w) for a wide variety of nonmetallic crystals within the adiabatic local density approximation (ALDA) in good agreement with experiment. In particular in the low-frequency range no adjustment of the band gap obtained within the local density approximation (LDA) seems to be necessary. Relativistic effects on the dielectric response have been found to be important for a few semimetals that have inverted bandstructures within the LDA. Exchange-correlation effects beyond the ALDA have been treated by a polarization-dependent functional for the effective electric field, with improved dielectric functions as result.

Institut für Theoretische Physik, Universitt Würzburg, Am Hubland, D-97074, Würzburg, Germany,

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

no abstract available

Organic and Molecular Inorganic Chemistry, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

promotor prof.dr.B.Feringa, supervisor dr.P.Th.van Duijnen, 2001

No abstract available

No abstract available

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

No abstract available

Faculty of Applied Physics, Computational Optics, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands

We have calculated the reflectance anisotropy for the GaAs (110) surface using the discrete cellular method. This method extends the range of application of standard discrete dipole calculations by incorporating nonlocal polarizabilities. The method adds a second quantum mechanical channel of nonlocality, which turns out to be necessary and yields very good agreement between theory and experiment.

We give an overview of the underlying concepts of time-dependent density-functional theory. The basic relations between densities, potentials and initial states, for time-dependent many-body systems are discussed. We obtain some new results concerning the invertability of response functions. Some fundamental difficulties associated with the time-dependent action principle are discussed and we show how these difficulties can be resolved by means of the Keldysh formalism.

In this paper we treat the dominant relativistic effects in the optical response properties of mercury selenide using time-dependent density-functional theory. The scalar relativistic effects have been included within the zeroth-order regular approximation (ZORA) in both the ground-state DFT calculations, and in the time-dependent response calculations. Within this approximation the HgSe crystal is found to be a semimetal. In a previous study (J. Chem. Phys. 114, 1860 (2001)) we have shown that TDDFT/ZORA can be applied successfully to narrow-gap semiconductors, such as indium antimonide, that become semimetallic within the local density approximation when scalar relativistic effects are included. Results are given for the band structure, the static dielectric constant and the dielectric function of HgSe, and these results are compared with the similar ones for InSb. We find considerably improved results for the dielectric function of HgSe when relativistic effects are included.

The third- and fifth-order time-domain Raman responses of liquid carbon disulfide have been calculated, taking local field effects into account through the dipole-induced dipole approximation to the polarizability. The third-order response is shown to be in excellent agreement with experimental data. The calculated two-dimensional shape of the fifth-order response is compared with recently reported experimental observations of what is claimed to be pure fifth-order response. Considerable discrepancies are observed which might be explained by contamination of the experimental results with sequential and especially parallel third-order cascaded Raman response. A new choice of polarization conditions is proposed, which increases the discrimination against these unwanted cascading effects, as compared to the previously discussed fully polarized and magic angle conditions.

In this paper we show how relativistic effects can be included in the time-dependent density-functional theory for the optical response properties of nonmetallic crystals. The dominant scalar relativistic effects have been included using the zeroth-order regular approximation (ZORA) in the ground-state DFT calculations, as well as in the time-dependent response calculations. We show that this theory can also be applied to indium antimonide in the zinc-blende structure, not withstanding the fact that it turns into a semi-metal when scalar relativistic effects are included. Results are given for the bandstructure, the static dielectric constant and the dielectric function, for the various levels on which relativity can be included, i.e. non-relativistic, only in the ground-state, or also in the response calculation. Comparisons of our calculated results are made with experiment and other theoretical investigations. With the inclusion of scalar relativistic effects, the bandstructure of InSb becomes semi-metallic within the local density approximation and we find a deviation of 5% from experiment for the static dielectric constant. Also the dielectric function is improved and the spectra are in good agreement with experiment, althought the spectral features are shifted to somewhat lower energies compared to experiment.

Theoretical Chemistry, and Solid State Physics, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

In this paper time dependent density functional theory (TDDFT) calculations of excited state polarizabilities of conjugated molecules are presented. The increase in polarizability upon excitation was obtained by evaluating the dependence of the excitation energy on an applied static electric field, the excitation energy was found to vary quadratically with the field strength. The excess polarizabilities obtained for singlet excited states are in excellent agreement with experimentally obtained values for short oligomers. For longer oligomers the excess polarizability is considerably overestimated, similar to DFT calculations of ground state polarizabilities. Excess polarizabilities of triplet states were found to be smaller than those for the corresponding singlet state. This also agrees with experimental results. Negative polarizabilities are observed for the lowest singlet A_{g} states which is caused by the quadratic Stark effect. All results are explained in terms of a sum-over-states description for the polarizability.

In this paper we present a new approach to calculate optical spectra, which for the first time uses a polarization dependent functional within current density functional theory (CDFT), which was proposed by Vignale and Kohn [Phys. Rev. Lett. 77, 2037 (1996)]. This polarization dependent functional includes exchange-correlation (xc) contributions in the effective macroscopic electric field. This functional is used to calculate the optical absorption spectrum of several common semiconductors. We achieved in all cases good agreement with experiment.

Experiment and Simulation

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands,

Department of Organic Chemistry,Indian Institute of Science, Bangalore 560012, India

Time-resolved resonance Raman spectroscopy has been used to study the structure of the triplet excited state of bromanil. These experimental results were then simulated using parameters from density functional theoretical calculations and wave packet dynamics, in order to understand the structure and mode-specific displacements of the resonant excited state. The transition dipole moments and the energy separation of the T_{1} and T_{n} states were obtained from time-dependent DFT calculations. We have demonstrated application of the technique to tetrabromo-*p*-benzoquinone. From our calculations, the observed T_{1} >T_{n}absorption spectrum has been assigned to the * ^{3}B_{g} > ^{3}A_{u}* transition. The geometry has been optimized for the resonant higher triplet state, T

A new charge analysis is presented that gives an accurate description of the charge distribution in molecules. The method is generally applicable to any method able to provide atomic multipole moments, but in this paper we take advantage of the way the Coulomb potential is calculated within the Density Functional Theory framework. We investigated a set of 31 molecules as well as all amino acids to test the quality of the method and found accurate results for the molecular multipole moments directly from the DFT calculations, as well as the values represented by the charges. The deviations from experimental values for the dipole/quadrupole moments are also small.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

We present the theoretical and technical foundations of the Amsterdam Density Functional (ADF) program with a survey of the characteristics of the code (numerical integration, density fitting for the Coulomb potential, and STO basis functions). Recent developments enhance the efficiency of ADF (e.g., parallelisation, near order-N scaling, QM/MM) and its functionality (e.g., NMR chemical shifts, COSMO solvent effects, ZORA relativistic method, excitation energies, frequency-dependent (hyper) polarisabilities, atomic VDD charges). In the Applications section we discuss the physical model of the electronic structure and the chemical bond, i.e. Kohn-Sham molecular orbital (MO) theory, and illustrate the power of the Kohn-Sham MO model in conjunction with the ADF-typical fragment approach to quantitatively understand and predict chemical phenomena. We review the "activation-strain transition state" (ATS) interaction model of chemical reactivity as a conceptual framework for understanding how activation barriers of various types of (competing) reaction mechanisms arise and how they may be controlled. Finally, we include a brief discussion of an application of time-dependent density functional theory (TDDFT) to indicate how this development further reinforces the ADF tools for the analysis of chemical phenomena.

Metallo Proteins, Gorlaeus Laboratories, University of Leiden, Einsteinweg 55, 2333 CC Leiden, The Netherlands

No abstract available

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands,

Materials Research Department, Risø National Laboratory, DK-4000 Roskilde, Denmark

In the present work, we have calculated the static and frequency-dependent polarizability tensors for a series of single-walled boron nitride nanotubes and compared with corresponding results for carbon nanotubes. The calculations have been performed by employing a dipole-dipole interaction model based on classical electrostatics and an Unsöaut;ld dispersion formula. In comparison, we have carried out *ab intio* calculations at the SCF level of the static polarizability of the smaller nanotubes with the STO-3G basis set. For the frequency-dependent polarizability of C_{60} we found excellent agreement between the most accurate SCF calculations in the literature, the interaction model and experimental results. In particular, the frequency-dependence is modelled accurately indicating that the interaction model is a useful tool for studying the frequency-dependence of materials. For the nanotubes, we observe the same trends in the interaction model and in the SCF STO-3G results when the number of atoms is increased. However, the values obtained with the interaction model are about 100% larger than the corresponding SCF STO-3G results, due to the small size of the STO-3G basis set. We also find that the boron nitride nanotubes have smaller magnitudes of the polarizability tensor components than the corresponding components for the carbon nanotubes with the same geometry and number of atoms. Furthermore, we find that the geometry of the tube has a large influence on the anisotropy of the polarizability components, whereas the mean polarizability remains almost unaffected when the geometrical configuration is modified. Finally, we observe a relatively small frequency-dependence of the polarizability tensor of BN nanotubes.

Department of Organic Chemistry,Indian Institute of Science, Bangalore 560012, India,

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The triplet excited state of tetrabromo-p-benzoquinone has been studied for the first time using time-resolved resonance Raman experiments. Density functional theoretical calculations have been carried out on the ground and triplet excited states. The molecular orbitals, geometries and vibrational frequencies have been analysed. Computed normal modes have been used to carry out normal mode analysis to obtain the potential energy distribution of all the vibational frequencies. Observed bands in the triplet state spectrum have been assigned using measured depolarization ratios, comparison with other quinones and calulated spectra. Changes in the geometry of the parent quinone on substitution with bromine has been correlated to the changes in the characters of the frontier molecular orbitals. Increased participation of the halogen lone pairs leads to significant changes in the geometry of the ground and excited states. It is found that the effect of halogen substitution is more pronounced on the excited state than on the ground state.

A Direct Reaction Field study

Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands,

The polarization of the excited states of near-perpendicularly twisted ethylene in the condensed phase has been investigated by means of Direct Reaction Field (DRF) calculations. In these calculations, five organic solvents with variable polarity and polarizability were simulated by 50 discrete, classically described solvent molecules. The excited states of near-perpendicular ethylene were described using ab initio methods at the CISD level of theory using a DZV basis set.

It is demonstrated that there is a distinct correlation between the polarity of the solvent and the occurrence and stabilization of charge separated excited states of ethylene. Large dipole moments were observed for ethylene excited states in polar solvents, indicating that an asymmetric distribution of polar solvent molecules around the ethylene can introduce enough symmetry breaking to cause charge separation. This behaviour was not observed for (models of) non-polar solvents. This charge separation process can be designated as unbiased 'sudden polarization' since the solvent shells used were in equilibrium with the non-polarized ethylene solute.

The dynamics of a two-electron system in a strong laser pulse is described by the time-dependent extended Hartree-Fock (TDEHF) scheme. Ionization yields for a one-dimensional helium model are calculated and compared with results of exact calculations (full correlation) and time-dependent Hartree-Fock calculations (no correlation). The knee-structure in the double ionization curve appears also for the TDEHF calculations, but the yields are more than an order of magnitude too low. The total ionization probability agrees well with the exact results and much improved results for the single ionization yields are obtained. Due to the reduced dimensionality of the model, the TDEHF ground state consists of a left and right orbital. While one of the orbitals easily transforms into a continuum state, the other remains localized and screens the nucleus. In this manner, even the one-dimensional orbitals can be considered to be *inner* and *outer* orbitals.

Towards a Model Hamiltonian

We present a density functional and time dependent density functional study of the ground, ionic and excited states of a series of oligomers of Thiophene. We show that, for the physical properties, the most relevant HOMO and LUMO molecular orbitals develop gradually from the monomer molecular orbitals into occupied and unoccupied broad bands in the large length limit. We show that the band gap and ionization potentials decrease with size as found experimentally and from empirical calculations. This gives credence to the simple tight binding model Hamiltonian approach for these systems. We demonstrate that the length dependence of the experimental excitation spectra for both the singlet and triplet excitations can be very well explained with an extended Hubbard like Hamiltonian with a monomer on site coulomb and exchange interaction and a nearest neighbor coulomb interaction. We also study the ground state and excited state electronic structure as function of the torsion angle between the units in a dimer and find almost equal stability for the transoid and cisoid isomers, with a transition energy barrier for isomerization of only 4.3 kcal/mol. Fluctuations in the torsion angle turn out to be very low in energy and therefore of great importance in describing even the room temperature properties. At a torsion angle of 90 degrees the hopping integral is switched off for the HOMO levels because of symmetry, allowing a first principles estimate of the on-site minus the next neighbor Coulomb interaction as it enters in a Hubbard like model Hamiltonian.

We present results of *ab-initio* electronic structure calculations of Mn core-valence and *d-d* transitions in LaMnO_{3}. The results are important for the analysis of recent X-ray absorption and anomalous X-ray scattering experiments at the Mn *K*-edge in LaMnO_{3},. We compare on-site *1s* to *3d* excitations with excitations to the *3d* shell of adjacent Mn ions and find that the first two peaks of the pre-edge region correspond respectively to majority-spin and minority-spin *e _{g} (3d)* states on neighboring Mn ions. For on-site

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands,

Instituto Nazionale per la Fisica della Materia, Universit Cattolica, Via Trieste 17, 25121 Brescia, Italy

no abstract available

Theory

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

A subdivision of space into discrete cells underlies the traditional discrete dipole method. This model presumes that nonlocal electric interactions between cells only are sufficient to describe the electromagnetic response of a condensed matter system. This is realistic for simple dielectrics, but is not otherwise. Cells can also influence each other directly through the wave functions, if they extend across cell boundaries. In general such nonlocal quantum mechanical interaction results in the ocurrence of nonlocal polarizabilities. In this paper it is shown how existing discrete dipole descriptions of finite systems, slabs and (semi-)infinite systems have to be altered to incorporate the effects of nonlocal polarizabilities. The modified method is called the discrete cellular method.

Department of Chemistry, University of Auckland, Private Bag 92019, Auckland, New Zealand

The point-charge model for the nuclear quadrupole moment (PCNQM) was successfully applied to atoms and linear molecules for picture-change-error-free determination of electric field gradients (EFGs). From these EFGs accurate values for the nuclear quadrupole moment could be obtained in combination with spectroscopical data. In this work we will present an extension of this model to systems with low or absent symmetry. At the C_{1} system of 1-fluoro-1-chloropropane we set up the formalism for the asymmetric PCNQM model. In cases where larger molecules with more than one heavy atom are considered, correlated Dirac-Fock calculations are still extremely expensive and methods which use approximate relativistic treatments or reduction to two components will find intensive use. Especially the Douglas-Kroll method is a very accurate approximation to the Dirac-Fock case and within the PCNQM formalism only operators of Coulomb-type are introduced allowing for a picture-change-error free description of EFGs for these molecules.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

We discuss ways to obtain analytical gradients within the scalar Zeroth Order Regular Approximation to the Dirac-Fock equation within an *ab inito* context. Simply employing the relativistic density within the non-relativistic gradient package is in error by 10^{-5}. We introduce a new strictly atomic scheme which in addition to yielding exact gradients is also computationally inexpensive and avoids the gauge invariance problems that plague molecular ZORA approaches. We show that the total and orbital energies produced with the scaled version of this method are generally, i.e. except for very short interatomic distances, very close to the full molecular scaled ZORA results. Equilibrium geometries from full molecular scaled ZORA and strictly atomic ZORA are shown to be within 0.01 A from Dirac-Fock.

no abstract available

no abstract available

Development and Application of Parallel and Order-N DFT Methods

No abstract available

Fachbereich Chemie, Philipps-Universitaet Marburg, Hans-Meerwein-Strae, D-35032 Marburg, Germany,

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Up till now, there has been a significant disagreement between theory and experiment regarding hydrogen bond lengths in Watson-Crick base pairs. To investigate the possible sources of this discrepancy, we have studied numerous model systems for adenine-thymine (AT) and guanine-cytosine (GC) base pairs at various levels (i.e., BP86, PW91 and BLYP) of nonlocal density functional theory (DFT) in combination with different Slater-type orbital (STO) basis sets. Best agreement with available gas-phase experimental A-T and G-C bond enthalpies (-12.1 and -21.0 kcal/mol) is obtained at the BP86/TZ2P level, which (for 298 K) yields -11.8 and -23.8 kcal/mol. However, the computed hydrogen bond lengths show again the notorious discrepancy with experimental values. The origin of this discrepancy is not the use of the plain nucleic bases as models for nucleotides: the disagreement with experiment remains no matter if we use hydrogen, methyl, deoxyribose or 5'-deoxyribose monophosphate as the substituents at N9 and N1 of the purine and pyrimidine bases, respectively. Even the BP86/DZP geometry of the Watson-Crick-type dimer of deoxyadenylyl-3',5'-deoxyuridine including one Na^{+} ion (with 123 atoms our largest model for sodium adenylyl-3',5'-uridine hexahydrate, the crystal of which had been studied experimentally with the use of X-ray diffraction) still shows this disagreement with experiment. The source of the divergence turns out to be the molecular environment (water, sugar hydroxyl groups, counterions) of the base pairs in the crystals studied experimentally. This has been missing, so far, in all theoretical models. After we had incorporated the major elements of this environment in our model systems, excellent agreement between our BP86/TZ2P geometries and the X-ray crystal structures was achieved.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The complexity of the electronic absorption spectrum of NO_{2} can be attributed to a conical intersection of the potential energy surfaces of the two lowest electronic states, the electronic ground state of ^{2}A_{1} symmetry and the first electronically excited state of ^{2}B_{2} symmetry. In a previous paper we reported on the feasibility of using the hyperfine splittings, specifically the Fermi-contact interaction, to determine the electronic ground state character of the excited vibronic states in the region just above the conical intersection; 10,000 to 14,000 cm^{-1} above the electronic ground state. High-resolution spectra of a number of vibronic bands in this region were measured by exciting a supersonically cooled beam of NO_{2} molecules with a narrow-band Ti:Sapphire ring laser. The energy absorbed by the molecules was detected by the use of a bolometer. In the region of interest rovibronic interactions play no significant role, with the possible exception of the vibronic band at 12,658 cm^{-1}, so that the fine- and hyperfine structure of each rotational transition could be analyzed by using an effective Hamiltonian. In the previous paper we restricted ourselves to an analysis of transitions of the K_{-} = 0 stack. In the present paper we extend the analysis to transitions of the K_{-}=1 stack, from which, in addition to hyperfine coupling constants, values of the A rotational constants of the excited NO_{2} molecules can be determined. Those rotational constants also contain information about the electronic composition of the vibronic states, and, moreover, about the geometry of the NO_{2} molecule in the excited state of interest. The results of our analyses are compared with those obtained by other authors. The conclusion arrived at in our previous paper that determining Fermi-constants is useful to help characterize the vibronic bands, is corroborated. In addition, the A rotational constants correspond to geometries that are consistent with the electronic composition of the relevant excited states as expected from the Fermi-constants.

Time-dependent density functional theory has been used to calculate the static and frequency-dependent dielectric function of non-metallic crystals. We show that a real-space description becomes feasible for crystals by using a combination of a lattice-periodic (microscopic) scalar potential with a uniform (macroscopic) electric field as perturbation in a periodic structure calculation. The induced density and microscopic potential can be obtained self-consistently for fixed macroscopic field by using linear response theory in which Coulomb interactions and exchange-correlation effects are included. We use an iterative scheme, in which density and potential are updated in every cycle. The explicit evaluation of Kohn-Sham response kernels is avoided and their singular behaviour as function of the frequency is treated analytically. Coulomb integrals are evaluated efficiently using auxiliary fitfunctions and we apply a screening technique for the lattice sums. The dielectric function can then be obtained from the induced current. We obtained for C, Si and GaAs within the adiabatic local density approximation in good agreement with experiment. In particular in the low-frequency range no adjustment of the LDA band gap seems to be necessary.

We present here the measurement of the single-polymer entropic elasticity and the single covalent bond force profile, probed with two types of atomic force microscopes (AFM) on a synthetic polymer molecule: polymethacrylic acid in water. The conventional AFM allowed us to distinguish two types of interactions present in this system when doing force spectroscopic measurements: the first interaction is associated with adsorption sites of the polymer chains onto a bare gold surface, the second interaction is directly correlated to the rupture process of a single covalent bond. All these bridging interactions allowed us to stretch the single polymer chain and to determine the various factors playing a role in the elasticity of these molecules. To obtain a closer insight into the bond rupture process, we moved to a force sensor stable in position when measuring attractive forces. By optimizing the polymer length so as to fulfill the elastic stability conditions, we were able for the first time to map out the entire force profile associated with the cleavage of a single covalent bond. Experimental data coupled with molecular quantum mechanical calculations strongly suggest that the breaking bond is located at one end of the polymer chain.

A Finite Field MD method has been developed to calculate non-resonant Raman response. The method has been used to calculate the third- and fifth-order responses for liquid CS_{2}. From the third-order response the intensity of the third-order cascading processes, has been estimated. The calculated ratio between the true fifth-order intensity and the intensity of the third-order cascading processes supports experimental observations claiming the 2-dimensional Raman spectra to be dominated by the third-order cascading processes.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

In this paper we present the implementation of the two component scaled ZORA method in the molecular electronic structure package GAMESS-UK. It is the first application of this method, which was earlier investigated in the context of Density Functional Theory, in molecular ab initio basis set calculations. The performance of the method is tested in atomic calculations, for which we can compare with numerical results, on Xenon and Radon and in molecular calculations on the molecules: AgH, HI, I_{2}, AuH, TlH and Bi_{2}. In calculations on the I_{2} molecule we investigated the effect on the orbital energies of the different approaches regarding the internal Coulomb matrix used in the ZORA method. For the remaining molecules we computed harmonic frequencies and bondlengths. It is shown that the scaled ZORA approach is a cost effective alternative to the Dirac-Fock method.

Integral generation, SCF and four-index transformation in the Dirac-Fock package MOLFDIR

Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands,

Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, USA

The treatment of relativity and electron correlation on an equal footing is essential for the computation of systems containing heavy elements. Correlation treatments that are based on four-component Dirac-Hartree-Fock calculations provide presently the most accurate, albeit costly, way of taking relativity into account. The requirement of having two expansion basis sets for the molecular wave function puts a high demand on computer resources. The treatment of larger systems is thereby often prohibited by the very large runtimes and files that arise in a conventional Dirac-Hartree-Fock approach. A possible solution for this bottleneck is a parallel approach which not only reduces the turn-around time but also spreads out the large files over a number of local disks. Here we present a distributed-memory parallelization of the program package MOLFDIR for the integral generation, Dirac-Hartree-Fock and four-index MS transformation steps. This implementation scales best for large AO spaces and moderately sized active spaces.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The ab initio scalar ZORA/IORA approach, which was previously tested within the context of numerical and basis set SCF calculations, is generalised to include electron correlation. The technical details of the method are investigated in calculations on the systems: Ne_{2}, Ar_{2}, Kr_{2}, Xe_{2} and AgH. For the weakly bonded rare gas dimers we calculated the bond lengths and well depths using the non-relativistic, ZORA, scaled ZORA and IORA MP2 method. The relativistic effect on the potential energy mininum, obtained with the most accurate method (scaled ZORA), is shown to account for the deviations between the best non-relativistic results and experiment.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Ab initio molecular orbital methods at the CBS-Q level of theory have been used to study the effect of substituent (F, Cl, NH_{2}, OH and CH_{3}) on the gas-phase acidities of formic acid, HCOOH its silicon and sulfur derivatives R- M(=X)XH(M = C, Si; X = O, S; R = F, Cl, OH, NH_{2}, and CH_{3}). For formic acid and its thio and dithio derivatives the acidity changes upon substitution are irregular and depend on both the type of substituent, possition and degree of replacement of oxygen atoms by sulfur atoms. For sila carboxylic acids and their thio and dithio derivatives the calculated acidities regularly increase in the order: R SiOOH < R Si(=S)OH << R Si(=O) SH < R SiSSH, (R = H, F, Cl, OH, NH_{2}, and CH_{3}). The chloro derivatives are the strongest among the sila acids studied. The highest gas phase acidity (1277.6 kJ mol-1) has been calculated for ClC(=S)OH.

Faculty of Applied Physics, Computational Optics, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands

We demonstrate that the description of the optical reflectance anisotropy of GaAs(110) requires a complete microscopic treatment of both surface and bulk, which is feasible in the discrete cellular method. This method is an extension of standard discrete dipole calculations and accounts for nonlocality in the electrodynamical local fields and ab-initio nonlocal polarizabilities. The results of our calculations are in excellent agreement with experiment and we show that the anisotropy is surface induced.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

We present results of ab initio calculations for d-d transitions, which arise in the mid-infrared spectrum of undoped cuprate compounds. It has been suggested that these transitions arise at energies as low as 0.4 eV in La_{2}CuO_{4} and Sr_{2}CuO_{2}Cl_{2}. We study the differences in d-d transition energies in a series of cuprates that contains compounds in which the Cu ions are sixfold, fivefold or fourfold coordinated. Furthermore, we analyze the dependence of the 3d_{x2-y2} to 3d_{z2} excitation energy on the ratio of the in plane and apex copper-ligand distances in the model system CuO. Our cluster calculations do not support the assignment of the 0.4 - 1 eV band to phonon and magnon sidebands of a d-d transition. On the other hand, we confirm the interpretation of the peak around 1.7 eV observed in CuGeO_{3} as arising from d-d transitions.

The dielectric function of a range of non-metallic crystals of various lattice types is studied by means of a real-space and full-potential time-dependent density functional method within the adiabatic local density approximation. Results for the dielectric constant (at optical frequencies) are given for crystals in the sodium chloride, the fluorite, the wurtzite, the diamond and the zincblende lattice structure. The frequency-dependent dielectric function for the crystals in the diamond and zincblende lattice structure are also presented. We compare our calculated results with experimental data and other theoretical investigations. Our results for the dielectric constants are in good agreement with the experimental values. The accuracy of the results is comparible to the one which is commonly found for TDDFT calculations on molecular systems, typically with a deviation of 3-5% from experiment. The spectral features of the dielectric functions appear in the calculations at somewhat lower energies compared to experiment.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands,

Instituto Nazionale per la Fisica della Materia, Universit Cattolica, Via Trieste 17, 25121 Brescia, Italy

Ab initio theoretical results for the 2p and 3p hole states of an Mn^{2+} ion are reported in order to determine the importance of atomic contributions to the XPS spectra of bulk MnO. A combined treatment of relativity and electron correlation reveals important physical effects that have been neglected in virtually all previous work. The many body and relativistic effects included in the atomic model are able, without any ad hoc empirical parameters, to explain most of the features of the MnO XPS spectra. In particular, it is not necessary to invoke charge transfer to explain the complex p-level spectra

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

In this paper we present the first application of the ZORA (Zeroth Order Regular Approximation of the Dirac Fock equation) formalism in Ab Initio electronic structure calculations. The ZORA method, which has been tested previously in the context of Density Functional Theory, has been implemented in the GAMESS-UK package. As was shown earlier we can split off a scalar part from the two component ZORA Hamiltonian. In the present work only the one component part is considered. We introduce a separate internal basis to represent the extra matrix elements, needed for the ZORA corrections. This leads to different options for the computation of the Coulomb matrix in this internal basis. The performance of this Hamiltonian and the effect of the different Coulomb matrix alternatives is tested in calculations on the radon en xenon atoms and the AuH molecule. In the atomic cases we compare with numerical Dirac Fock and numerical ZORA methods and with non relativistic and full Dirac basis set calculations. It is shown that ZORA recovers the bulk of the relativistic effect and that ZORA and Dirac Fock perform equally well in medium size basis set calculations. For AuH we have calculated the equilibrium bond length with the non relativistic Hartree Fock and ZORA methods and compare with the Dirac Fock result and the experimental value. Again the ZORA and Dirac Fock errors are of the same order of magnitude.

In the Direct Reaction Field (DRF) approach to the description of events in the condensed phase, quantum parts (QM) are embedded in a (semi-)classical environment (MM). QM is described with any appropriate wavefunction, while MM is modeled with point charges and interacting polarizabilities and/or a dielectric continuum, which may have finite ionic strength. The static and response potentials are made part of QM's Hamiltonian (hence Direct RF), leading to one- and two-electron contributions. Hence we obtain also a good estimate of the dispersion. For QM/MM and MM/MM interactions point charges and polarizabilities are treated as belonging to (model) charge distributions. The rest of the short range repulsion is accounted for by a model atom pair potential borrowed from CHARMM. The same model - with a Saltier-Quirked dispersion expression, based on the same interacting polarizabilities - leads to a definition of a classical, polarizable force field used in QM/MM and MM-only Monte Carlo simulations. Here we present calculated static (hyper-)polarizabilities a, b and g for some molecules in various environments.

Fachbereich Chemie, Philipps-Universitaet Marburg, Hans-Meerwein-Strae, D-35032 Marburg, Germany,

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The view that the hydrogen bonds in the Watson-Crick adenine-thymine (AT) and guanine-cytosine (GC) base pairs are in essence electrostatic interactions with substantial resonance assistance from the p electrons is questioned. Our investigation is based on a state-of-the-art density functional theoretical (DFT) approach (BP86/TZ2P) which has been shown to properly reproduce experimental data. Through a quantitative decomposition of the hydrogen bond energy into its various physical terms, we show that, at variance with widespread belief, donor-acceptor orbital interactions (i.e. charge transfer) in s symmetry between N or O lone pairs of one base and N-H s* acceptor orbitals on the other base do provide a substantial bonding contribution which is, in fact, of the same order of magnitude as the electrostatic interaction term. The overall orbital interactions are reinforced by a small p component, stemming from polarization in the p-electron system of the individual bases. This p component is, however, one order of magnitude smaller than the s term. Furthermore, we have investigated the synergism in a base pair between charge-transfer from one base to the other through one hydrogen bond and in the opposite direction through another hydrogen bond, as well as the cooperative effect between the donor-acceptor interactions in the s- and polarization in the p-electron system. The possibility of C-H....O hydrogen bonding in AT is also examined. In the course of these analyses, we introduce an extension of the Voronoi deformation density (VDD) method which monitors the redistribution of the s- and p-electron densities individually out of (DQ > 0) or into (DQ > 0) the Voronoi cell of an atom upon formation of the base pair from the separate bases.

Direct Reaction Field (DRF) model was developed for calculations of electronic properties of molecules in the condensed phases. In the DRF approach the electrons of (part(s) of a system is described with wave functions, the larger parts classically with point charges and polarizabilities. Leaving out the quantum mechanical part(s) leads naturally to a polarizable force field. In this paper we demonstrate the usefulness of the Direct Reaction Field (DRF) model for the study of many body interactions in polar systems. We have calculated the many body interactions in clusters of HF, H_{2}O and urea in our classical polarization model and compared the results to ab initio calculations using large basis sets. We find that the results obtained using the classical model compare excellently to ab initio results.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Ab initio molecular orbital calculations at the CBS-Q level of theory have been used to study the rotational conformers and acidity of dithiosilanoic acid and several of its derivatives R-SiSSH (R=H, F, Cl, NH_{2}, OH and CH_{3}). Vibrational spectra were evaluated at the ab initio MP2(Full)/6-31G(d) level of theory. For all six acids studied the syn conformers are predicted to have the lowest energy. The syn - anti enthalpy difference is varying between 4 and 7 kJ mol^{-1}. Dithiosilanoic acid is about 60 and 96 kJ mol^{-1} more acid than dithioformic and silanoic acid, respectively.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

New experimental results are reported on state-dependent steric effects in NO-Ar inelastic scattering. The NO molecules are selected in theJ=1/2- Lambda-doublet state of the electronic ground state and oriented relative to the incident Ar-atoms. The steric asymmetry,S=(s_{NO}-s_{ON})/(s_{NO}+s_{ON}) has been measured as a function of the final rotational state J'. In a previous study, quantum mechanical scattering calculations were found to predict strong oscillations in S, but experimental evidence for this behaviour was not conclusive. The results of recent experiments presented here provide clear evidence of the qualitative correctness of the theoretical calculations.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Time-dependent density functional theory provides a first principles method for the calculation of frequency-dependent polarizabilities, hyperpolarizabilities, excitation energies and many related response properties. In recent years, the molecular results obtained by several groups have shown that this approach is in general more accurate than the time-dependent Hartree-Fock approach, and is often competitive in accuracy with computationally more demanding conventional ab initio approaches. In this paper, our implementation of the relevant equations in the Amsterdam Density Functional program is described. We will focus on certain aspects of the implementation which are necessary for an efficient evaluation of the desired properties, enabling the treatment of large molecules. Such an efficient implementation is obtained by: using the full symmetry of the molecule, using a set of auxiliary functions for fitting the (zeroth- and first-order) electron density, using a highly vectorized and parallelized code, using linear scaling techniques, and, most importantly, by solving the response equations iteratively.

Starting from many-body perturbation theory we have constructed a new variational expression for the total energy of many-electron systems. This expression is a functional of two independent variables, the one-electron Green function and the screened Coulomb interaction. The new functional as well as a much older variational expression by Luttinger and Ward (LW) are tested on the interacting electron gas. Both functionals yield extraordinary accurate total energies although the new functional requires a much cruder input and is therefore easier to apply to more realistic systems.

Physics Department, Indian Institute of Technology, Powai, Bombay-400 076, India,

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands ,

Departimento di Chimica Fisica ed Inorganica, Universitá di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy

In the present note we outline a simple scheme for generating the Configuration Interaction matrix elements for spin-orbit interactions in molecules. The procedure leads to a close parallelism with spin-free permutation group approaches. Unitary shift operators have been successfully used on the orbital space to generate the matching permutations necessary to evaluate the required matrix elements. The procedure has been adequately illustrated using examples.

We present the results of detailed studies of the potential energy surfaces of the iodine-benzene charge-transfer complex obtained from (fully counterpoise corrected) ab initio calculations at the MP2 level, and from (semi-)classical calculations . The most stable conformations found were the above bond and the above carbon conformations. The axial conformation was found to be somewhat less stable. The remarkable difference in intermolecular distance for different orientations of the iodine is explained in terms of the polarization anisotropy. This feature makes the construction of an accurate classical force field rather difficult because of the marked dependence of the repulsion parameter - usually the radius - for iodine on both orientation and polarization of the iodine. Investigation of the oscillator strengths of different complex geometries shows that there are many conformations in which the charge transfer excitation can take place.

Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Relativistic time dependent density functional calculations have been performed on the excited states of the M(CO)_{6} (M = Cr, Mo, W) series. Our results, in agreement with previous DSCF [Pollak, 1997] and CASPT2 [Pierloot, 1996] calculations on Cr(CO)_{6}, indicate that in all members of the series the lowest excited states in the spectra do not correspond to ligand field (LF) excitations, as has been accepted in the past. Instead they correspond to charge transfer (CT) states. The LF excitations are calculated at much higher energy than suggested by the original assignment by Beach and Gray [Beach, 1968] and at different energy along the M(CO)_{6} series, being much higher in the heavier carbonyls than in Cr(CO)_{6}.

These results lead to a definitive reassessment of the role of the LF states in the photochemical dissociation of the metal-CO bonds in the M(CO)_{6} series, suggesting that the experimentally observed photodissociation of the M-CO bond upon irradiation into the lowest energy bands occurs in the heavier carbonyls, as it does in Cr(CO)_{6}, from CT and not from LF states. A comparison with the experimental data available and, in the case of Cr(CO)_{6}, also with high-level correlated ab initio calculations, [Pierloot, 1996] proves the reliability of the present TDDFT approach. The choice of the xc functional is found to have a large effect on the excitation energies, demonstrating that even for quite "normal", low-lying excitations the xc functional may play an important role. In the heavier carbonyls, mostly in W(CO)_{6}, relativistic effects are seen to be relevant for the LF states as well as for the CT states arising from the (2t_{2g})^{5}(3t_{2g})^{1} configuration.

Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands,

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands,

Department of Chemistry, University of Nevada, Las Vegas, Nevada 89154, USA,

Department of Chemistry, University of California, Santa Barbara, California 93106, USA

No abstract available

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

No abstract available

The electronic structure of NiO, with emphasis on the Ni 3s-hole ionic states, is studied using non-orthogonal configuration interaction, NOCI, wavefunctions for an NiO_{6} model of the crystal. Orbital sets are relaxed, or optimized, separately for each configuration used in the NOCI and orbital symmetry breaking, or localization, is allowed. This localization is important for configurations that involve large amounts of charge transfer from O(2p) to the Ni(3d) shell. The NOCI method insures an unbiased treatment of the relative energies of configurations that involve different degrees of charge transfer from O(2p) to Ni(3d). The use of fully relaxed orbitals is shown to be necessary to obtain accurate energies and intensities for core level ionic states observed with X-ray photoelectron spectroscopy, XPS. The NOCI energies and intensities for the lowest and first excited, high spin coupled, 3s-hole states are in good agreement with XPS spectra. Both high spin 3s-hole states are found to have significant, but partial, charge transfer character.

The polarizabilities of 15 organic molecules are calculated using the Restricted Hartree Fock (RHF) method, Density Functional Theory (DFT) and the Direct Reaction Field (DRF) approach. The RHF method gives rather poor results, while the other two give average deviations comparable to the experimental uncertainty. The DRF approach is very fast (< 1s), but underestimates the anisotropy of molecules containing pi-bonds. Three DFT methods were used (Local Density Approximation, Becke-Perdew, model potential) which need more time (9 - 80 hours) but give a better overall accuracy, which increases towards the basis set limit. The model potential improves the Becke-Perdew potential, which in turn gives better results than the Local Density Approximation.

A brief overview is presented of some theoretical work on the symmetry breaking of electronic wavefunctions that followed the early work of Bagus and Schaefer who observed that a considerable lower SCF energy could be obtained for an ionized state of the O_{2} molecule with a 1s hole if the symmetry restrictions were released, so that the core hole could localize on one of the two oxygens. In the present contribution some emphasis is put on the properties of symmetry adapted wavefunctions, which are obtained through a nonorthogonal CI amongst different symmetry broken wave functions. n-p* Excited states of para-benzoquinone and 3d-s ionized states of Cu_{2} are used as illustrative examples.

Molecular properties of the uranyl ion ([UO_{2}]^{2+}) are studied using both a non-relativistic and a relativistic method. Inclusion of relativity leads to a bond length expansion and makes the electric field gradient (EFG) at the uranium nucleus strongly dependent on the U-O bond distance. The non-relativistic EFG value is found to be much larger than the relativistic value. An analysis of the non-relativistic and relativistic wave functions is given and shows the presence of a so-called "U(6p) core-hole". A different ordering of the valence spinors is found compared to previous work. It is confirmed that the HOMO has su character and has a large U(5f) contribution.

A series of embedded cluster calculations have been performed to study the dependence of the calculated ionization and excitation energies in CuCl and NiO on the representation of the direct cluster surrounding. Different embedding schemes have been applied. First, a cluster of one TM ion plus its nearest counterions has been embedded in point charges only. Next, model potentials (AIEMPs) have been used to represent the first layer of ions around the basic cluster, and finally, we embedded the cluster in the charge distribution of frozen ions. The calculations show that the ionization and excitation energies change considerably when the Pauli repulsion of the cluster atoms with the direct surrounding is accounted for.

The Direct Reaction Field approach is briefly reviewed. Preliminary reports of the calculations on solvent induced shifts in the transition of acetone in various solvents, and the dissociation of ter-butyl chloride in water are given.

Institute for Chemical Reaction Science, Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-8577, Japan

The minimum energy conformations and racemization barriers for the chiral sterically overcrowded helical alkenes, trans- and cis-1,1',2,2',3,3',4,4'- octahydro- 4,4'-biphenanthrylidenes (1 and 2), are reported. The trans-1 and cis-2 isomers can each adapt three different conformations, (P,P) and (M,M) (an enantiomeric pair) and an achiral (P,M) meso form, of which only the chiral isomers were obtained by synthesis. The conformations and heats of formation of (M,M)-(E)-1, (P,M)-(E)-1, (M,M)-(Z)-2, and (P,M)-(Z)-2 isomers were determined by MOPAC AM1 calculations. The racemization process for both the trans- and cis- isomers is postulated to occur via the (P,M) isomers by two successive inversions of the cyclohexenyl ring; (M,M) to and from (P,M) to and from (P,P). The (M,M) to (P,M) and reverse (P,M) to (M,M) isomerizations were simulated by reaction path calculations, providing the molecular structure and the activation energy of the transition state for each isomerization. For each racemization process, the activation enthalpy (DH) was calculated as 23.9 and 19.9 kcal mol-1 for trans-olefin 1 and cis-olefin 2, respectively. These values reasonably agree with the experimental values obtained by temperature- dependent circular dichroism, optical rotation, and ^{1}H NMR magnetization transfer measurements: DH = 24.6 and 20.8 kcal mol-1 for trans-olefin 1 and cisolefin 2, respectively. While the racemization of cis-isomer 2 is controlled by the steric interaction of H5 with C4'a and C4'b, the surprisingly high barrier for trans-olefin 1 is due to the severe steric interaction between H5 and H3'a and/or H3'b protons.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The first time-dependent density functional theory (TDDFT) calculations on the spectra of molecules containing transition metals are reported. Three prototype systems are considered, of which the assignments are controversial: MnO_{4}- , Ni(CO)_{4}, and Mn_{2}(CO)_{10}. The TDDFT results are shown to be comparable in accuracy to the most elaborate ab initio calculations and lead to new insights in the spectra of these molecules. In some cases, the presented TDDFT results differ substantially, in both the ordering and the values for the excitation energies, from the older DFT method for the calculation of excitation energies: the DSCF approach. For the Mn_{2}(CO)_{10} molecule, the presented results are the highest-level theoretical results published so far. Over all, the results show that TDDFT can be a very useful tool in the calculation and interpretation of the spectra of transition metal compounds.

Department for Computation and Information, Daresbury Laboratory, Daresbury, Warrington WA4 4AD, United Kingdom,

SHELL International Oil Products, Badhuisweg 3, 1031 CM Amsterdam, The Netherlands

Proton-energy differences, ammonia adsorption, and D/H-exchange barriers for methane at selected isolated Brønsted sites in zeolites FAU, MFI, BEA, ERI, and CHA are studied by combined quantum-chemical-classical (QM/MM) calculations in an attempt to understand the factors that determine the reactivity at these Brønsted sites.

The barrier of the D/H-exchange reaction for methane was found to correlate well with the calculated ammonia chemisorption energy, but even better with the O-Al-O angle of the free zeolite Brønsted site the reaction is taking place on, provided the Si-O-Al-O-Si moiety over which the reaction takes place is more or less collinear. The barrier is considerably higher if this collinearity is weaker, which may be explained by the necessity of costly back-bone distortions to accommodate the geometrical requirements of the transition state. This is confirmed by similarly strong correlations with the O-Al-O angle change going from the free acid site to zeolite-ammonium-ion bidentate structures, which may be thought of as a measure of the back-bone distortion.

A new measurement of the D/H-exchange barrier in BEA is also reported. It was found to be 88 18 kJ/mol, lower than the experimental barriers in both FAU and MFI.

A recently proposed spin-restricted open-shell KohnSham ~ROKS! method is applied to investigate various atomic and molecular multiplet states. A wide range of multiplets is considered: multiplet terms for which the spin-restricted open-shell theory of Roothaan applies, as well as state situations which cannot be described by Roothaans theory ~e.g., states of square cyclobutadiene, etc.!. Problems associated with the use of approximate density functionals and possible perspectives of the ROKS method are discussed.

Institut für Theoretische Physik, Universitt Würzburg, Am Hubland, D-97074, Würzburg, Germany

We show that a time-dependent particle density *n( rt)* obtained from a given many-particle system can, under mild restrictions on the initial state, always be reproduced by an external potential

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands,

Laboratoire de Chimie Theorique Appliquee, Facultes Universitaires Notre-Dame de la Paix, Rue de Bruxelles, 61, B-5000 Namur, Belgium,

Department of Chemistry, University of California, Santa Barbara, California 93106, USA

Density functional calculations on the (non)linear optical properties of conjugated molecular chains using currently popular exchange-correlation (xc) potentials give overestimations of several orders of magnitude. By analyzing exactand Krieger-Li-Iafrate xc potentials, the error is traced back to an incorrect electric field dependence of the response part of the xc potential in local and gradient-corrected density approximations, which lack a linear term counteracting the applied electric field.

Mathematical Institute, University of Oxford, 24/29 St. Giles', OX3 7RY, Oxford, U.K.

No abstract available

Local transitions at the NiO (100) surface have been studied with ab initio calculations by means of CASSCF/CASPT2. In addition to the well known surface specific d-d transition at 0.6 eV also the recently proposed surface d-d transition at 2.1 eV is confirmed. The broad peak at 1 eV is proposed to consist of transitions to the surface ^{3}B_{2}, ^{3}A_{2}, and b^{3}E d^{8} states and the bulk ^{3}T_{2g} state. Furthermore, the relative energies of the local ligand to metal charge-transfer excitations have been investigated. The lowest local CT state in NiO (100) has been calculated to be about 2 eV lower than in bulk NiO.

Many experiments that one would like to describe theoretically have a common (idealised) form: one starts by perturbing the system one wants to study by an external agent (such as a laserpulse) and after a certain time interval one probes the system by measuring one of its dynamical variables such as its polarisation (dipole moment). In other words the dynamical response of the system to an external perturbation is measured. Often the system of interest, such as a liquid, is macroscopic in nature and it becomes impossible to describe the motion of all its individual constituents in full detail and one has to resort to the methods of statistical (quantum)mechanics. In this case it is often profitable to divide the system into two parts, a small subsystem (hereafter simply called "the system") which is described in full detail (e.g. a molecule in a liquid) and the rest of the system (called "the environment" or "the bath") which is treated only statistically and which interacts with the system proper. At the start of the experiment one assumes that the system and the bath are in stationary equilibrium and can be described by equilibrium thermodynamics. The external perturbation then excites the system in various ways, taking it out of statistical equilibrium. Subsequently the system interacts with the bath and will tend to loose (*dissipate*) its excess energy to its environment and will eventually return (*relax*) back to thermodynamic equilibrium. One can now study this relaxation process by measuring the value of some observable of the system as function of the delay since the system was excited (the dynamic response function of this observable), thus obtaining information about the system bath interaction (the intermolecular forces in a liquid for example). If the delay is long enough one expects the system to have relaxed to equilibrium and one simply measures the equilibrium value of the response (which usually vanishes, e.g. the average dipole moment of a molecule in a liquid is zero, due to random orientations).

In this short course we will discuss the theoretical tools which are needed to describe the type of experiment discussed above. There are apparently three ingredients we will have to treat which are absent in the description of the groundstate properties of isolated molecules.

- We will have to decide how to describe a quantum system statistically rather than by specifying its wavefunction.

- We will have to describe the time dependent interaction with the external perturbing agent and the subsequent influence of this perturbation on the properties of the system as a function of time.

- Finally we will have to study the interaction of the system with its environment and decide on how to model the relaxation processes introduced above.

We will start with a brief summary of the quantum description of isolated systems and then we will address each of these three problems in turn and study how they come together in the description of quantum statistical response.

A theoretical investigation of the crystal field excitations in CoO is presented. Special attention is given to the excitation energy of the ^{4}A_{2g} state. In recent experimental and theoretical studies an excitation energy around 3.1 eV was reported. This is in disagreement with the 2.1 eV deduced from optical spectroscopy data. After analyzing electron correlation effects, spin-orbit interactions and the material model to represent the CoO crystal we can confirm the interpretation of the optical data, not only for the ^{4}A_{2g} state, but also for all other low-lying crystal field excitations. Electron correlation effects are found to have a significant differential effect on the excitation energies, ranging from +0.3 eV to -0.6 eV. Spin-orbit interactions are less important, affecting the excitation energies by at most 0.05 eV. Finally, we discuss the effect of the Pauli-repulsion between the cluster ions and the first shell of ions around the cluster. This affects the excitation energies by a small, but significant, amount.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

If a polar molecule can be selected in a rotational state of definite parity, subsequent orientation of the molecule in an electric field mixes opposite parity states. The degree of mixing reflects the degree of orientation. Therefore, the intensity ratio of spectral lines that correspond to transitions starting from the two parity states being mixed, forms a sensitive and accurate probe of the molecular orientation. If saturation of the spectral lines of interest is avoided, the absolute degree of orientation can be determined, without recourse to other experimental data but line intensities. The method is illustrated for the case of the NO molecule.

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Institute of Physics, University of Szczecin, Wielkopolska 15, 70-451 Szczecin, Poland,

Chemistry Department, Odense University, Campusvej 55, DK-5230 Odense M, Denmark

The effect of relativity on the properties of the interhalogens ClF, BrF, BrCl, IF, IBr and IBr is studied by comparing relativistic and non-relativistic calculations. Bond lengths, harmonic frequencies and dissociation energies show that the bond is weakened in the relativistic formalism. Relativity increases the electric dipole moment whereas the electric quadrupole moment and dipole polarizability display an irregular behaviour. The relativistic contributions to the electric dipole and quadrupole moment of the iodine containing molecules are 10-20% of the total value whereas the contributions in the other molecules cannot be neglected. The value of the electric quadrupole moment is dominated by the relativistic contributions.

Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands,

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands,

Department of Chemistry, University of Nevada, Las Vegas, Nevada 89154, USA,

Department of Chemistry, University of California, Santa Barbara, California 93106, USA

DFT schemes based on conventional and less conventional exchange-correlation (XC) functionals have been employed to determine the polarizability and second hyperpolarizability of p-conjugated polyacetylene chains. These functionals fail in one or more of several ways:

i) the correlation correction to a is either much too small or in the wrong direction, leading to an overestimate;

ii) g is significantly overestimated;

iii) the chain length dependence is excessively large, particularly for g and for the more alternant system; and,

iv) the bond length alternation effects upon g are either underestimated or qualitatively incorrect. The poor results with the asymptotically correct van Leeuwen-Baerends XC potential show that the overestimations are not related to the asymptotic behaviour of the potential. These failures are described in terms of the separate effects of the exchange and the correlation parts of the XC functionals. They are related to the short-sightedness of the XC potentials which are relatively insensitive to the polarization charge induced by the external electric field at the chain ends.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

An accurate determination of frequency-dependent molecular hyperpolarizabilities is at the same time of possible technological importance and theoretically challenging. For large molecules, Hartree--Fock theory was until recently the only available ab initio approach. However, correlation effects are usually very important for this property, which makes it desirable to have a computationally efficient approach in which those effects are (approximately) taken into account. We have recently shown that frequency-dependent hyperpolarizabilities can be efficiently obtained using time-dependent density functional theory. Here, we shall present the necessary theoretical framework and the details of our implementation in the Amsterdam Density Functional program. Special attention will be paid to the use of fit functions for the density and to numerical integration, which are typical of density functional codes. Numerical examples for He, CO, and *para*-nitroaniline are presented, as evidence for the correctness of the equations and the implementation.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

In this paper we present time-dependent density functional calculations on frequency-dependent first (b) and second (g) hyperpolarizabilities for the set of small molecules, N_{2}O, CO_{2}, CS_{2}, C_{2}H_{4}, NH_{3}, CO, HF, H_{2}O, and CH_{4}, and compare them to Hartree--Fock and correlated ab initio calculations, as well as to experimental results. Both the static hyperpolarizabilities and the frequency dispersion are studied. Three approximations to the exchange-correlation (xc) potential are used: the widely used Local Density Approximation (LDA), the Becke--Lee--Yang--Parr (BLYP) Generalized Gradient Approximation (GGA), as well as the asymptotically correct Van Leeuwen--Baerends (LB94) potential. For the functional derivatives of the xc potential the Adiabatic Local Density Approximation (ALDA) is used. We have attempted to estimate the intrinsic quality of these methods by using large basis sets, augmented with several diffuse functions, yielding good agreement with recent numerical static LDA results. Contrary to claims which have appeared in the literature on the basis of smaller studies involving basis sets of lesser quality, we find that the static LDA results for b and g are severely overestimated, and do not improve upon the (underestimated) Hartree--Fock results. No improvement is provided by the BLYP potential which suffers from the same incorrect asymptotic behavior as the LDA potential. The results are however clearly improved upon by the LB94 potential, which leads to underestimated results, slightly improving the Hartree--Fock results. The LDA and BLYP potentials overestimate the frequency dependence as well, which is once again improved by the LB94 potential. Future improvements are expected to come from improved models for asymptotically correct exchange-correlation potentials. Apart from the LB94 potential used in this work, several other asymptotically correct potentials have recently been suggested in the literature and can also be expected to improve considerably upon the relatively poor LDA and GGA results, for both the static properties and their frequency dependence.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The visible absorption spectrum of NO_{2} is very dense and irregular, and shows signs of a chaotic frequency and intensity distribution in the higher energy region. The complexity of the spectrum is related to a conical intersection of the potential energy surfaces of the two lowest electronic states. Above the conical intersection strong vibronic interactions lead to hybrid eigenstates, which can be viewed as mixtures of low vibrational levels of the electronically excited state and high vibrational levels of the electronic ground state. As a contribution to the elucidation of the nature of the vibronic bands of NO_{2} we have measured high-resolution spectra of a number of vibronic bands in the region between 10,000 to 14,000 cm^{-1}, by exciting a supersonically cooled beam of NO_{2} molecules with a narrow-band Ti:Sapphire ring laser. The energy absorbed by the molecules was detected by a bolometer, and in some cases, laser-induced fluorescence was detected. The hyperfine structure is dominated by the Fermi-contact interaction and the magnitude of this interaction is a direct measure of the (electronic) composition of the hybrid eigenstates. In the region studied, rovibronic interactions appear to be insignificant. The fine- and hyperfine structure of each rotational transition can be analyzed by using an effective Hamiltonian approach. In the present paper we have restricted our analysis to transitions of K_= 0 stacks. The composition of the hybrid eigenstates is compared with ab initio calculations reported in the literature, leading to the conclusion that measurements of the hyperfine structure are a helpful tool in characterizing vibronic bands.

Department of Theoretical Physics, University of Lund, Sölvegatan 14A, S-22362 Lund, Sweden

The double-ionization yield of He is calculated with a one-dimensional fully correlated two-electron model for the low laser frequency of recent experiments. Results for a higher laser frequency also indicate a comparable very high double-ionization yield for sufficiently short pulses. It is shown that the Hartree - Fock approximation fails dramatically in describing the two-electron dynamics. Also, in a density functional theory approach, we demonstrate the need for an improved exchange correlation potential and for more accurate density functionals for the ionization probabilities.

Thole's modified dipole interaction model for constructing molecular polarizabilities from effective, isotropic atomic polarizabilities is reviewed and extended. We report effective atomic polarizabilities for H, C, N, O, S, and the halogen atoms, independent of their chemical environment. They are obtained by fitting the model both to experimental and calculated molecular polarizabilities, the latter to enable one to model ab initio polarizabilities for various basis sets.

The Direct Reaction Field model was used to calculate the solvent shift of the n to p* transition of acetone in eight different solvents. The computed shifts correspond excellently to experimental values. We found that dispersion interactions are an essential part of the model for correctly describing the shifts in both polar and apolar solvents. Improving the quality of the basis set generally improves the results, mainly due to an increase in electrostatic interactions.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Response calculations in the framework of time-dependent density functional theory (TDDFT) have by now been shown to surpass time-dependent Hartree--Fock (TDHF) calculations in both accuracy and efficiency. This makes TDDFT an important tool for the calculation of frequency-dependent (hyper-)polarizabilities, excitation energies and related properties of medium-sized and large molecules. Two separate approximations are made in the linear DFT response calculations. The first approximation concerns the exchange-correlation (xc) potential, which determines the form of the Kohn--Sham orbitals and their one-electron energies, while the second approximation involves the so-called xc kernel fxc, which determines the xc contribution to the frequency-dependent screening. By performing calculations on small systems with accurate xc potentials, constructed from {\it ab initio} densities, we can test the relative importance of the two approximations for different properties and systems, thus showing what kind of improvement can be expected from future, more refined, approximations to these xc functionals. We find that in most, but not all, cases, improvements to V_{xc} seem more desirable than improvements to f_{xc}.

We resolve an existing paradox regarding the causality and symmetry properties of response functions within time-dependent density-functional theory. We do this by defining a new action functional within the Keldysh formalism. By functional differentiation the new functional leads to response functions which are symmetric in the Keldysh time contour parameter, but which become causal when a transition to physical time is made. The new functional is further used to derive the equations of the time-dependent optimized potential method.

Reaction of pyridinyl-2-phosphonyl dichloride (6) with 1-phenyl-2,2-dimethylpropane-1,3-diol (9) leads to the two epimeric 2-oxo-2-(2-pyridinyl)- 4-phenyl-5,5-dimethyl- 1,3,2-dioxaphosphonnanes (10a,b). These can be separated and the stereochemistry assigned on the basis of 31P NMR spectroscopy. For 10a the pyridinyl substituent is arranged axially at phosphorus. Arguments derived from 2D NMR experiments indicated that the nitrogen of pyridine is locked in a conformation whereby the pyridinyl nitrogen points over the six-membered ring; in other words it is locked between the two ring oxygen substituents. This conclusion is substantiated by an X-ray crystal determination. Oxidation of 10a with hydrogen peroxide leads to the N-oxide (12). The crystal structure of 12 reveals that despite serious steric overcrowding the N-O bond is also oriented over the six-membered ring. Methylation of 10a with methyl trifluoromethanesulfonate affords the N-methyl pyridinium salt (13). NMR experiments indicate that in this case the methylated nitrogen has turned "outside" of the six-membered ring. The borane adduct of 10a appears on the basis of NMR data to have a conformation wherein the complexed borane is located just outside of the six-membered ring. Although crystal structures have not been obtained the pyridinyl-2-thiophosphonates (15a,b) obtained from treatment of 10a and 10b with [(4-MeOC_{6}H_{4})_{2}PS]_{2} appear to have the same conformational properties as 10a and 10b. Restricted Hartree-Fock geometry optimizations have been carried out to aid in clarifying this unexpected conformational behaviour. These calculational results are in excellent accord with the experimental observations, and provide insight into the reasons for the conformational behaviour.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

One of the most important steps in a Kohn-Sham type DFT calculation is the construction of the matrix of the Kohn-Sham operator (the "Fock" matrix). It is desirable to develop an algorithm for this step that scales linearly with system size. We discuss attempts to achieve linear scaling for the calculation of the matrix elements of the exchange-correlation and Coulomb potentials within a particular implementation (the ADF code) of the KS method. In the ADF scheme the matrix elements are completely determined by a 3D numerical integration, the value of the potentials in each grid point being determined with the help of an auxiliary function representation of the electronic density. Nearly linear scaling for building the total Fock matrix is demonstrated for systems of intermediate size (in the order of 1000 atoms). For larger systems further development will be desirable for the treatment of the Coulomb potential.

The vibronic band at 13352 cm^{-1} of NO_{2} is measured up to the hyperfine structure by bolometric detection.The excited vibronic state involved is a hybrid state having a major contribution from the electronic ground state. This follows from the hyperfine splitting of the K- = 0 and K- = 1 stacks as well as from the value of the rotational constant A'. The anomalous intensity distribution induced by rovibrational interactions is not observed in the vibronic band analyzed here. Although the authors suggest the absence of rovibronic interactions of significant strength, the authors underline the presence of vibronic interactions in the vibronic band analyzed here.

The superexchange interaction, parameterized by J, of bulk NiO and the NiO[100] surface was investigated using ab initio quantum chemical techniques. J Is influenced by two opposing mechanisms; the reduced Madelung potential at the surface leads to a small increase, whereas the change in the nickel coordination from six to five oxygens gives a larger decrease. As a result J at the [100] surface is predicted to decrease by about 20% with respect to the bulk. This contradicts another recent prediction to the effect that J should increase by about 50%. Experimental data are not yet available.

In this letter a non-orthogonal configuration interaction study is presented for the high spin coupled final states involved in the Ni 3s X-ray photoelectron spectra of NiO. Charge transfer effects play an important role in the interpretation of this spectrum. These effects are incorporated in a conceptually attractive way by a non-orthogonal configuration interaction description, in which the wave functions are expressed as linear combinations of the reference configuration and relaxed charge transfer configurations. The energy separation and relative intensities of the high spin coupled final states were calculated in good agreement with experiment. Both final states have considerable charge transfer character.

An approximation scheme was developed for the Kohn-Sham exchange-correlation potential v_{xc}, making use of a partitioning of v_{xc} into a long-range screening v_{scr} and a short-range response v_{resp} component. For the response part, a model v^{mod}_{resp} was used, which represents v_{resp} as weighted orbital density contributions, the weights being determined by the orbital energies. v^{mod}_{resp} possesses the proper short-range behavior and the atomic-shell stepped structure characteristic for v_{resp}. For the screening part, two model potentials v^{mod}_{scr} were used, one with the accurate Slater potential; the other one with the generalized gradient approximation (GGA) for the exchange part. Both use the GGA for the Coulomb correlation contribution to v_{scr} . The scheme provides an adequate approximation to v_{xc} in the outer-valence region with both the proper asymptotics and a rather accurate estimate of the ionization potential from the highest one-electron energy and a reasonable estimate of atomic E_{xc} and total energies E_{tot}.

A series of non singular two-component relativistic Hamiltonians is derived from the Dirac Hamiltonian by first performing the free-particle Foldy-Wouthuysen transformation and then a block-diagonalizing transformation. The latter is defined in terms of operators which can be determined iteratively through arbitrary order in c, leading to transformed Hamiltonians with the two-component block accurate through a^{2k}, k = 1, 2, 3, .... These Hamiltonians give relativistic energies which differ from Dirac's energies only in terms higher than a^{2k}. Their relation to other non singular methods of relativistic quantum chemistry (the Douglas-Kroll method, the regular Hamiltonian schemes) is discussed. By removing the spin-dependent operators, the derived Hamiltonians can be written in spin-free one-component form. The computational effort involved is essentially the same as in the case of the Douglas-Kroll scheme and amounts to relatively easy modification of the core Hamiltonian.

The results are reported of ab initio calculations on the magnetic ordering in NiO, a prototype of the antiferromagnetic insulator. By analyzing wave functions for different cluster models, information was obtained about the physical effects determining the sign and the magnitude of the magnetic coupling parameter J. The role of the edge oxygens, surrounding the essential unit (Ni_{2}O), is quantitatively important but purely environmental in contrast to the role of the bridging oxygen. Also, the importance of electron correlation and the usefulness of pseudopotentials in the calculations was studied. The final result of J compares reasonably with experiment (.apprx.50%), and possible sources for the remaining discrepancies are discussed.

The generalized gradient-approximated (GGA) energy functionals used in density functional theory (DFT) provide accurate results for many different properties. However, one of their weaknesses lies in the fact that Van der Waals forces are not described. In spite of this, it is possible to obtain reliable long-range potential energy surfaces within DFT. In this paper, we use time-dependent density functional response theory to obtain the Van der Waals dispersion coefficients C_{6}, C_{7} and C_{8} (both isotropic and anisotropic). They are calculated from the multipole polarizabilities at imaginary frequencies of the two interacting molecules. Alternatively, one might use one of the recently-proposed Van der Waals energy functionals for well-seperated systems, which provide fairly good approximations to our isotropic results. Results with the local density approximation (LDA), Becke-Perdew (BP) GGA and the Van Leeuwen-Baerends (LB94) exchange-correlation potentials are presented for the multipole polarizabilities and the dispersion coefficients of several rare gases, diatomics and the water molecule. The LB94 potential clearly performs best, due to its correct Coulombic asymptotic behavior, yielding results which are close to those obtained with many-body perturbation theory (MBPT). The LDA and BP results are systematically too high for the isotropic properties. This becomes progressively worse for the higher dispersion coefficients. The results for the relative anisotropies are quite satisfactory for all three potentials, however.

The character of the low-lying excited states of diatomic CuCl is studied primarily by means of the complete active space SCF (CASSCF) method and a second order perturbation approach with the CASSCF wave function as reference state [complete active space perturbation theory to second order (CASPT2)]. For comparison, the lower levels of the spectra of the Cu^{+} ion are also analyzed. A first order treatment of the scalar relativistic effects, the mass-velocity and Darwin terms, is included in the calculations. The importance of spin-orbit interactions is investigated by comparing our nonrelativistic valence shell CI (VCI) and relativistic results obtained with our four-component program suite MOLFDIR. The six lowest excited states of the CuCl molecule, which are related to the Cu^{+}(3d^{9}4s^{1})Cl^{-}(3s^{2}3p^{6}) ionic configuration, are assigned. The assignments agree with earlier theoretical work. Where they can be compared, the calculated spectroscopic constants are in good agreement with the experimental data.

The electronic structure, spectroscopic and bonding properties of the ground, excited and ionized states of iodine are studied within a 4-component relativistic framework using the MOLFDIR program package. The properties of the ^{1}S_{g}+^{ }ground state, calculated up to the CCSD(T) level of theory, are in good agreement with experiment. It is shown that relativistic effects and core-valence correlation need to be included in order to get results close to experiment and that the Breit interaction can be neglected. The lowest ionized states properties like the bond length, the dissociation energy and the harmonic frequency are studied at different levels of theory. The photoelectron spectrum and the potential energy curves of the ionized and excited states are calculated. The calculated properties of the excited states are in good agreement with experimental data and theoretical results of Teichteil and Pelissier [Chem. Phys. 180, 1 (1994)]. An alternative assignment of some recently measured, low lying, ionized states is proposed.

Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

The gas-phase He(I) photoelectron spectra of the MF_{4}(M = Ti, Zr, Hf) molecules have been recorded for the first time. Assignment of the spectra was achieved with the aid of ab initio molecular orbital and density functional theory (DFT) calculations. The spectra are found to be similar for each molecule, with the exception that the A^{2}T_{2}, ionic state in HfF_{4} is split by spin-orbit interaction. The measured splitting (0.18 0.03) eV, is in very good agreement with the value obtained from a relativistic DFT calculation, 0.16 eV, and arises from a small contribution from the Hf 5p orbital into the upper t_{2}, molecular orbital of HfF_{4}.

Theo Thole's Ph.D. work was the basis for the Direct Reaction Field method for incorporating a semi-classical "solvent" in quantum chemical calculations. The early stages of his work and later progress is reviewed, and a typical example of his analytical and programming skills-so far unpublished-is given.

Thole's interacting polarizability model to calculate molecular polarizabilities from interacting atomic polarizabilities is reviewed and its majorapplications in computational chemistry illustrated. The applications include prediction of molecular polarizabilities, use in classical expressions for intermolecular interactions for the computation of binding energies of molecular dimers and solvation (free) energies, and solvent effects in combined quantumchemical-classical (QM/MM) calculations. The examples demonstrate the wide applicability of the model, which is due to its firm foundation in the perturbation theory of intermolecular interactions, from which the polarizability emerges as one of the material properties determining the interaction between species. The true power of the model is its generality, rendering transferability to all sorts of chemical problems almost a non-issue.

After a word of welcome some personal remarks are made on Theo's start in theoretical chemistry, on his remarkable achievements thereafter and on his very special qualities as a friend and colleague.

The detailed reaction pathways for the hydration of carbon dioxide by water and water clusters containing two, three, and four water molecules (CO_{2} + nH_{2}O -> H_{2}CO_{3} + (n-1 )H_{2}O, n = l-4) have been investigated in both gas phase and aqueous solution using ab initio molecular orbital (MO) theory up to the quadratic configuration interaction QCISD(T)/6-31G(d,p)//MP2/6-31G(d,p) level, both SCRF and PCM models of continuum theory, and a mixed approach based on MO calculations in conjunction with Monte Carlo and reaction field simulations. It is confirmed that the CO_{2} hydration constitutes a case of active solvent catalysis where solvent molecules actively participate as a catalyst in the chemical process. In aqueous solution the hydration mechanism is multimolecular, where geometric parameters of the solvent fully intervene in the reaction coordinate. The hydration reaction was found to proceed through an attack of a water oxygen to the CO_{2} carbon in concert with a proton transfer to a CO_{2} oxygen. The proton transfer is assisted by a chain of water molecules, which is necessary for a proton relay between different oxygens. Owing to a significantly larger charge separation in the transition structures, nonspecific electrostatic interactions between solute and solvent continuum also play a more important stabilizing role. Regarding the answer to the title question, our calculations suggest that although a water tetramer (n = 4) seems to be necessary for CO, hydration in the gaseous phase, a reaction channel involving formation of a bridge containing three water molecules (n = 3) is likely to be actively involved in the neutral hydration of CO_{2} in aqueous solution.

Femtosecond pump-probe spectroscopy is used to explore the excited-state dynamics of TPE in polar and nonpolar solvents. Four excited states are shown to play an important role: the vertically excited S1 state, the Franck-Condon relaxed S1 state, a twisted charge-resonance state (in the literature often referred to as biradical), and a charge-separated state. The subpicosecond dynamics are dominated by a very large Stokes shift, which is primarily due to ultrafast elongation of the ethylenic C-C bond, and by wave packet motion of the phenyl-ring bending modes. On picosecond time scales, isomerization dynamics and charge separation by symmetry breaking occur. The latter process is made possible by an avoided crossing between the singly and doubly excited states of TPE, leading to a dramatic enhancement of the polarizability. The electron transfer across the C-C bond follows an adiabatic reaction path on the lower potential energy surface. In nonpolar solvents, an equilibrium is established with a symmetric charge resonance state, by thermally activated recrossing to the upper potential surface. In polar solvents this process is suppressed by solvent stabilization of the dipolar, zwitterionic form of TPE.

Departament de Química Física, Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain

The spectral transitions and the character of the low-lying excited states of the copper halides, CuX (X= Cl, Br, I) are studied by means of two different relativistic computational approaches. One is based on the CASSCF/CASPT2 approach with operators accounting for scalar relativistic effects evaluated as a first order correction to the CASSCF energy. The other is a fully relativistic four component SCF-CI treatment based on the Dirac-Coulomb hamiltonian and hence accounts intrinsically for spin-orbit coupling as well as for scalar effects. The lowest excited states (^{1, 3}S^{+}, ^{1, 3}P, ^{1, 3}D) are all closely related to the formal ionic configuration Cu^{+}(3d^{9}4s^{1}) X-(ns^{2}np^{6}). The agreement between calculated and measured transition energies and transition dipoles and their trends in the series strengthens recent assignments of the observed bands. Unobserved 'neutral' states, dominated by the configuration Cu(3d^{10}4s^{1}) X(ns^{2}np^{5}), are situated mostly far above the 'ionic' states. Particular attention was given to the mixing of these states i.e. to the importance of charge transfer effects in the description of the observed states. These seem to be of significance only for the ^{1}S^{+} states, judging from the weights of the charge transfer configurations in the total wave functions and the character of the open shell orbitals. The calculated increase in charge transfer on going from Cl to I in the series goes together with an increase in the calculated transition dipoles for the ^{1}S^{+} states. This is consistent with the observed decrease of the lifetimes. The magnitudes of the spin-orbit splittings in the ionic states are governed by the splitting in Cu^{+} (2000 cm^{-1}) as expected.

Reaction of YCl_{3}.THF_{3.5} with 2 equiv of [Me_{2}Si(NCMe_{3})(OCMe_{3})]Li produces [Me_{2}Si(NCMe_{3})(OCMe_{3})]_{2}Y-(-Cl)_{2}Li.THF_{2} (**1**), which easily loses LiCl to give [Me_{2}Si(NCMe_{3})(OMe_{3})]_{2}-YCl.THF (**2**). Salt metathesis of **2** with LiBH_{4}, LiOAr (OAr = O-2,6-(CMe_{3})_{2}C_{6}H_{3}), NaN(SiMe_{3})_{2}, and LiCH(SiMe_{3})_{2} gives the corresponding yttrium bis((alkoxysilyl)amido) derivatives, [Me_{2}Si(NCMe_{3})(OCMe_{3})]_{2}YR (R = BH_{4}.THF (**3**), OAr (**4**), N(SiMe_{3})_{2} (**5**), CH(SiMe_{3})_{2} (**6**)). The alkyl compound **6** reacts with H_{2} in THF to give an unstable hydride {[Me_{2}Si(NCMe_{3})(OCMe_{3})]_{2}Y(-H)}_{2} (**7**), which was identified by 1H NMR as a symmetric dimer in solution. Isolation of the hydride **7** appeared not to be possible; the disproportionation product, [Me_{2}Si(NCMe_{3})(OCMe_{3})]_{3}Y (**8**), was obtained instead. With HC-CR, **6** undergoes protolysis of both the alkyl and the (alkoxysilyl)amido ligands to yield {Y(C-CR)_{3}}_{n} for R = SiMe_{3} (**9**) and CMe_{3} (**10**). In contrast, polymerization to polyphenylacetylene was observed for R = Ph. Compound **6** reacts with N=CMe with metalation of the methyl group under proton transfer to the alkyl ligand to give CH_{2}(SiMe_{3})_{2}. Insertion of another N=CMe into the new Y-C bond and 1,3-H shift produces {[Me_{2}Si(NCMe_{3})(OCMe_{3})]_{2}Y((N,N')-NH-CMe=CH-C=N)}_{2} (**11**). The molecular structures of **6** and **11** show that the bis(N,O-bis(tert- butyl)(alkoxydimethylsilyl)amido) ligand system is slightly more bulky than the bis(pentamethylcyclopentadienyl) ligand set in compounds Cp*_{2}YR. A ROHF INDO/1 semiempirical molecular orbital study on a stripped and symmetrized model of **6**, [H_{2}Si(NH)(OH)]_{2}YCH_{3}, shows that the electronic properties of the bis((alkoxysilyl)amido) ligand system are quite different from those of [C_{5}H_{5}]_{2}YCH_{3} but compare well with those of the bis(benzamidinato) anaiogue [HC(NH)_{2}]_{2}YCH_{3}. The (alkoxysilyl)amido ligand binds dominantly through a strong, ionic Y-N bond, while the ether function coordinates only weakly. Like in the bis(benzamidinato)yttrium system, the (alkoxysilyl)amido and the alkyl ligands accumulate negative charge, resulting in essentially ionic compounds. This high ionicity makes the compounds have little tendency to engage in d-bond metathesis reactions and (catalytic) insertion chemistry. Because of the absence of charge delocalization within the (alkoxysilyl)amido ligands, these behave as strong Brønsted bases and compete successfully with the Y-C bond in C-H bond activation reactions.

Theoretical Chemistry, Katholieke Universiteit Nijmegen, Nijmegen, The Netherlands

In this paper we first describe the implementation of the zeroth order regular approximation (ZORA) in our density functional program for extended systems. We have found an approximation that reduces the cost of the relativistic calculations. Secondly, we present the outcome of nonrelativistic, scalar relativistic, and spin- orbit calculations on the adsorption energy of CO on the (111) surfaces of Ni, Pd, and Pt. We have examined simultaneously the effect of different exchange-correlation (XC) functionals, i.e. the local-density approximation (LDA) versus the generalized gradient approximation (GGA) and spin-compensated versus spin- polarized variants of these functionals.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Existence of collective effects in magnetic coupling in ionic solids is studied by mapping spin eigenstates of the Heisenberg and exact nonrelativistic Hamiltonians on cluster models representing KNiF_{3}, K_{2}NiF_{4}, NiO, and La_{2}CuO_{4}. Ab initio techniques are used to estimate the Heisenberg constant J. For clusters with two magnetic centers, the values obtained are about the same for models having more magnetic centers. The absence of collective effects in J strongly suggests that magnetic interactions in this kind of ionic solids are genuinely local and entangle only the two magnetic centers involved.

Experimental as well as theoetical values for the frequency-dependent hyperpolarizability of C_{60} differ by orders of magnitude. The authors present the first density functional calculation of a molecular frequency-dependent hyperpolarizability. Their implementation is very economical, enabling the treatment of molecules of this size in a potentially much more accurate way than can be obtained with alternative methods. Their results strongly support the recent results by L. Geng and J.C. Wright, who report much lower experimental values than previous authors.

1. Metoprolol, indoramine, codeine, tamoxifen and prodipine, compouds which are clinically used, and MDMA (ecstasy) were fitted in a small molecular model for substrates of human cytochrome P 4502D6.

2. For both the R- and S-enantiomer of metoprolol, the R- and S-enantiomer of MDMA, and for indoramine and codeine (all proven substrates of cytochrome P 4502D6) an acceptable fit in the substrate model was obtained.

3. For tamoxifen, for which the involvement of cytochrome P 4502D6 in the 4-hydroxylation is uncertain, no acceptable fit could be obtained in the substrate model.

4. For prodipine, a competitive inhibitor of P 4502D6, for which the involvement of P 4502D6 in the metabolism is uncertain, no acceptable fit in the substrate model could be obtained.

5. The substrate model was extended in a direction in which two large known substrates extend from the original substrate model. This extension did not change the flat hydrophobic region of the original substrate model.

The polarization behavior of the low lying excited states in the vicinity of perpendicularly twisted (D_{2d}) ethylene has been investigated in a quantum mechanical CISD approach, in which the quantum system was embedded in a polarized dielectric continuum modeling a non-symmetric distribution of the solvent around the solute. The results show a strong polarization in the two lowest lying excited states in the region where the two vacuum energy surfaces of those states intersect, which strongly suggests that the lowering of the symmetry of the solvent shell can provide the adiabatic coupling for the avoided crossing between both potential energy surfaces. All examined values of e, with the exception of the smallest investigated value (e = 2.0), showed that the polarization remains intact on progressive twisting towards the perpendicular geometry.

The partly filled 3d shell in solid transition metal compounds is quite localized on the transition metal ion and gives rise to large electron correlation effects. With the recently developed CASSCF/CASPT2 approach electron correlation effects can be accounted for efficiently. The CASSCF step accounts for the non-dynamical correlation and part of the dynamical correlation, the following CASPT2 step takes largely care of the remaining dynamical correlation in a perturbative way. This approach is applied to the d-d excitations in NiO for which both non-dynamical and dynamical electron correlation effects have substantial influence on the energy differences. Excitation energies that compare well to the experimental data are obtained and the importance of the different electron correlation effects can be assessed.

A two-parameter form of the Dirac Hamiltonian, which follows from the authors' earlier papers and can be treated in the framework of double perturbation theory, is derived. The usual 1/c^{2} perturbation expansion is partitioned into a partial expansion in terms of 1/c^{2} and additional expansion into a series of the metric perturbation parameter l^{2}. The block diagonalization of the two-parameter Dirac Hamiltonian corresponds to infinite summation with respect to the first of these parameters and leads to what is known as the 2-component Hamiltonian of the zeroth-order regular approxn. (ZORA, CPD). When corrected through first order in l^{2} the ZORA Hamiltonian reproduces exactly all terms of the Pauli approximation. The block diagonalized Hamiltonian can be further modified to a form which has the non-relativistic energy operator as the leading term. However, this form has been found to possess several inconvenient features which are remedied by initializing the perturbation expansion with the ZORA Hamiltonian. This stresses the importance of the variation solution of the ZORA equation as the lowest order approximation for the expansion in the metric perturbation parameter l^{2}.

A new procedure for constructing double group symmetry functions is presented. Using this method integrals over Hermitian operators can become real quantities, even though the integrand and the functions themselves are complex. This is especially of interest to 4-component relativistic methods that use the Dirac-Coulomb Hamiltonian directly in electronic structure calculations.

The authors calculate the Heisenberg exchange J in the quasi-2D antiferromagnetic cuprates La_{2}CuO_{4}, YBa_{2}Cu_{3}O_{6}, Nd_{2}CuO_{4} and Sr_{2}CuO_{2}Cl_{2}. The authors apply all-electron (MC)SCF and nonorthogonal CI calculations to [Cu_{2}O_{11}]^{18-}, [Cu_{2}O_{9}]^{14-}, [Cu_{2}O_{7}]^{10-} and [Cu_{2}O_{7}Cl_{14}]^{14-} clusters in a model charge embedding. The (MC)SCF triplet and singlet ground states are well characterized by Cu^{2+} (d_{x2-y2}) and O^{2-}. The antiferromagnetic exchange is strongly enhanced by admixing relaxed (MC)SCF triplet and singlet excited states, in which a single electron is transferred from the central O ion to Cu. The authors ascribe this effect to orbital relaxation in the charge transfer component of the wavefunction. Close agreement with experiment was obtained.

Recently, the 1st density functional theory (DFT) calculations of Raman intensities and depolarization ratios were published. Those calculations were done in the static approxn. Here, the authors use time-dependent DFT to include the dependence of those properties on the frequency of the exciting light wave. By analytically calculating the frequency-dependent polarizability at different nuclear positions, the approach is closer to a fully analytic one than the previous DFT studies. Results for five diatomics improve upon previous TDHF (time dependent Hartree-Fock) and SOPPA (second order polarization propagator approximation) ab initio results and show that the frequency dependence cannot be ignored in quantitative comparisons to experiment. Results for the important Q-branch differential Raman cross section of N_{2} are closer to the experimental value than previous results. Inclusion of the frequency dependence has little effect on the depolarization ratios, but improves the results for the cross sections obtained in static DFT calculations. Results obtained with three different exchange-correlation potentials yield similar results.

The first direct experimental evidence is reported of large orientational (head-tail) effects in rotationally inelastic collisions, for the specific case of NO and Ar. NO is selected in the ^{2}P_{1/2} j = (1/2)^{-} state and oriented in an electric field. The steric effect, measured for collisions to three distinct final rotational states, appears to depend on the j' value and the parity of the final state. This behavior also follows from quantum-mechanical scattering calculations, but the calculated values do not coincide with the corresponding experimental ones.

No abstract available

The solvent shift of the n to p* transition of acetone in H_{2}O, MeCN, and tetrachloromethane was calculated in a combined quantum mechanical-classical mechanical approach, using both dielectric continuum and explicit, polarizable molecular solvent models. The explicit modeling of solvent polarizability allows for a seperate analysis of electrostatic, induction, and dispersion contributions to the shifts. The calculations confirm the qualitative theories about the mechanisms behind the blue shift in polar solvents and the red shift in nonpolar solvents, the solvation of the ground state in polar solvents and the red shift in nonpolar solvents, the solvation of the ground state due to electrostatic interactions being preferential in the former, and favorable dispersion interaction with the excited state, in the latter case. Good quantitative agreement for the solvent shift between experiment (+1,700, +400, and -350 cm^{-1} in H_{2}O, MeCN, and tetrachloromethane, resp.) and the explicit solvent model (+1,821, +922, and -381 cm^{-1}) was reached through a modest Monte Carlo sampling of the solvent degrees of freedom. A consistent treatment of the solvent could only be realized in the molecular solvent model. The dielectric-only model needs reparameterization for each solvent.

An analysis of the structure of the optimized effective Kohn-Sham exchange potential v_{x} and its gradient approximations is presented. The potential is decomposed into the Slater potential v_{s} and the response of v_{s} to density variations, v_{resp}. The latter exhibits peaks that reflect the atomic shell structure. Kohn - Sham exchange potentials derived from current gradient approaches for the exchange energy are shown to be quite reasonable for the Slater potential, but they fail to approximate the response part, which leads to poor overall potentials. Improved potentials are constructed by a direct fit of v_{x} with a gradient-dependent Pad approximant form. The potentials obtained possess proper asymptotic and scaling properties and reproduce the shell structure of the exact v_{x}.

It is shown how the regularized two-component relativistic Hamiltonians of Heully et al. and Chang, Pelissier, and Durand can be viewed as arising from a perturbation expansion that unlike the Pauli expansion remains regular even for singular attractive Coulomb potentials. The performance of these approximate Hamiltonians is tested in the framework of the local density approximation and the relation of their spectrum to that of the Dirac Hamiltonian is discussed. The circumstances under which the current approximations are superior to the Pauli Hamiltonian are analyzed. Finally, it shown how the Hamiltonians could be used within the context of conventional Hartree-Fock theory.

The electronic structures and bonding of UF_{6} and UF_{6}^{-} are studied within a relativistic framework using the MOLFDIR program package. A stronger bonding but more ionic molecule is found if one compares the relativistic with the nonrelativistic results. The first peak in the photoelectron spectrum of Karlsson, et al., is assigned to the 12g_{8u} component of the 4t_{1u} orbital, in agreement with other theoretical and experimental results. Good agreement is found between the experimental and theoretical 5f spectrum of UF_{6}^{-}. Some properties, like the dissociation energy and electron affinity, are calculated, and the necessity of a fully relativistic framework is shown. The Breit interaction has an effect on the core spinors and the spin-orbit splitting of these spinors but the influence on the valence spectrum is negligible.

The direct reaction field (DRF) force field gives a classical description of intermolecular interactions based on ab initio quantum-chemical descriptions of matter. The parameters of the DRF force field model molecular electrostatic and response properties, which are represented by distributed changes and dipole polarizabilities. The advantages of the DRF force field is that it can be combined transparently with quantum-chemical descriptions of a part of a large system, such as a molecule in solution or an active site in a protein. In this study, the theoretical basis for the deviation of the parameters is reviewed, paying special attention to the four interaction components: electrostatic, induction, dispersion, and repulsion. The ability of the force field to provide reliable intermolecular interactions is assessed, both in its mixed quantum-chemical-classical and fully classical usage. Specifically, the description of the water dimer and the solvation of water in water is scrutinized and seen to perform well. The force field is also applied to systems of a very different nature, viz. the benzene dimer and substituted-benzene dimers, as well as the acetonitrile and tetrachloromethane dimers. Finally, the solvation of a number of polar solutes in water is investigated. It is found that as far as the interaction energy is concerned, the DRF force field provides a reliable embedding scheme for molecular environments. The calculation of thermodynamic properties, such as solvation energy, requires better sampling of phase space than applied here.

The molecular Kohn-Sham exchange-correlation potential v_{xc} and the energy density e_{xc} have been constructed from ab initio first- and second-order density matrices for the series XH (X=Li, B, F). The way various effects of electronic structure and electron correlation manifest themselves in the shape of v_{xc} and e_{xc} has been analyzed by their decomposition into various components; the potential of the exchange-correlation hole, the kinetic component and (in the case of v_{xc}) the ``response'' component. The kinetic energy of noninteracting particles T_{s}, the kinetic part of the exchange-correlation energy T_{c}, and the energy of the highest occupied molecular orbital N have been obtained with reasonable accuracy and the effect of bond formation on these functionals has been studied.

A benchmark study of a number of relativistic correlation methods is presented. Bond lengths, harmonic frequencies, and dissociation energies of the molecules F_{2}, Cl_{2}, Br_{2}, I_{2} and At_{2} are calculated at various levels of theory, using both the Schrödinger and the Dirac-Coulomb-(Gaunt) Hamiltonian.

A benchmark study of a number of four-component relativistic correlation methods is presented. Bond lengths, harmonic frequencies, and dissociation energies of the molecules HF, HCl, HBr, HI, and HAt are calculated at various levels of theory, using both the Schrödinger and the Dirac-Coulomb-(Gaunt) Hamiltonian. The inclusion of relativity leads to a weakening of the bond, giving a decrease in the calculated harmonic frequencies and dissociation energies of the hydrogen halides. The effect on the bond length is small. These trends are explained by considering the relativistic change in hybridization induced by the spin-orbit coupling.

It is shown that it is possible to construct, within the framework of the basis set expansion method, the full Foldy-Wouthuysen transformation (i.e., to all orders in the inverse velocity of light) for an arbitrary potential once the Dirac equation has been solved. On this basic an iterative procedure to solve the Dirac equation is suggested that involves only the large component, obviating the time-consuming (at least in molecular calculations) introduction of large basis sets for a proper description of just the small components. The methods are used to compare the expectation value of radial distance operator in the Dirac picture and in the Shroedinger picture for the orbitals of the Uranium atom.

The exchange-correlation potentials .nu.xc which are currently fashionable in density functional theory (DFT), such as those obtained from the local density approximation (LDA) or generalized gradient approximations (GGAs), all suffer from incorrect asymptotic behavior. In atomic calculations, this leads to substantial overestimations of both the static polarizability and the frequency dependence of this property. In the present paper, it is shown that the errors in atomic static dipole and quadrupole polarizabilities are reduced by almost an order of magnitude, if a recently proposed model potential with correct Coulombic long-range behavior is used. The frequency dependence is improved similarly. The model potential also removes the overestimation in molecular polarizabilities, leading to slight improvements for average molecular polarizabilities and their frequency dependence. For the polarizability anisotropy, the authors find that the model potential results are not an improvement over the LDA and GGA results. The authors' method for calculating frequency-dependent molecules response properties within time-dependent DFT, which they described in more detail elsewhere, is summarized.

In this paper we will calculate the effect of spin-orbit coupling on properties of closed shell molecules, using the zero-order regular approximation to the Dirac equation. Results are obtained using density functionals including density gradient corrections. Close agreement with experiment is obtained for the calculated molecular properties of a number of heavy element diatomic molecules.

The formalism for a relativistic open-shell CCSD(T) method is presented and implemented in a computer program, RELCCSD. The code can be used for calculations with 2- or 4-component relativistic reference wave functions and allows a full inclusion of the spin-orbit coupling. The code is interfaced to the MOLFDIR program system. We illustrate its use with ab initio calculations of the fine structure splittings of Cl, FO, ClO, O_{2}^{+}, and O_{2}-. The triples correction is found to make a large contribution to the Cl atom splitting, which is within 23 cm^{-1}, of the experimental value. The molecular results are within 4 cm^{-1} of the experimental values where these are available. The value for FO is predicted to be -195 4 cm^{-1}, in good agreement with experiment.

We report spin-restricted and symmetry-RHF cluster calculations on the lower excited states of a Cu+ impurity in NaF in order to investigate their dependence on cluster size. In contrast to previous work on smaller clusters, we found all states arising from the configurations e_{g}^{4 }t_{2g}^{5 }a_{1g}^{1} and e_{g}^{3} t_{2g}^{6 }a_{1g}^{1} to be local. Delocalization can occur when an unbalanced choice of cluster and set of embedding point charges is made. These results confirm the local perspective from which previous spectroscopic assignments have been made. However, the inconsistency with the results from OD-EPR spectra remains.

Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

The basis set superposition error (BSSE) for the Cr(CO)_{5}-CO and (CO)_{5}Mn-Mn(CO)_{5} bonds is calculated for a large variety of STO basis sets. All investigated metal basis sets, being at least TZ for 3d and DZ for 4s are adequate. Ligand basis sets of TZD quality or better are required in order to have a low BSSE (6-8 kJ/mol or less) for both the metal-ligand and the metal-metal bond. When the ligand s, p basis set is of double-zeta quality, the BSSE is significant for the metal-ligand bond (16-22 kJ/mol depending on the polarization functions), but it is partly canceled by a basis set incompleteness error of opposite sign. For the metal-metal bond, the BSSE for DZ s, p ligand bases is unacceptably large (33-57 kJ/mol), leading to much too high bond energies if no correction for BSSE is applied. In general, the bond energies after correction for BSSE are rather stable. It is remarkable that for the metal-metal bond, but not for the metal-ligand bond, there is for all pure s, p ligand bases after correction for the BSSE a discernible basis set incompleteness error (ca. 15 kJ/mol), which only disappears after adding at least one polarization function. Agreement of the converged results for both geometries and bond energies with experiment is excellent. Cr(CO)_{6}.

The synthesis of [PhC(NSiMe_{3})_{2}]_{2}Y(-Cl)_{2}Li.2THF (**1**) from YCl_{3}3.5THF and [PhC(NSiMe_{3})_{2}]Li, which is easily transformed into [PhC(NSiMe_{3})_{2}]_{2}YCl.THF (**2**), provides a useful entry into the chemistry of several bis(N,N'-bis(trimethylsilyl)benzamidinato)yttrium complexes. Those prepared from **2** by chloride metathesis include [PhC(NSiMe_{3})_{2}]_{2}YR (R = BH_{4}.THF (**3**), N(SiMe_{3})_{2} (**4**), 2,6-(CMe_{3})_{2}-4-MeOC_{6}H_{2} (**5**), (-Me)_{2}Li.TMEDA (**6**) (TMEDA = N,N,N',N'-tetramethylethylenediamine), CH_{2}Ph.THF (**7**), CH(SiMe_{3})_{2} (**8**)). Similar to **8** [p-MeOC_{6}H_{4}C(NSiMe_{3})_{2}]_{2}YCH(SiMe_{3})_{2} (**8 _{OMe}**) could be prepared starting from [p-MeOC

We present an exact equation for the exchange-correlation potential of time-dependent density functional theory. This relation is derived using a many-particle Green's function formalism due to Keldysh. We furthermore show how this equation can be derived from an action principle. The method presented provides a systematic way to derive correlation contributions to the time-dependent exchange-correlation potential.

No abstract available

We solve the Dirac equation by solving the two-component energy-dependent equation for the large component that results from the elimination of the small component. This requires for every occupied orbital the diagonalization of a Hamiltonian. Advantages are, however, that these Hamiltonians are all bounded from below, unlike the Dirac Hamiltonian, and that only a basis set for the large component is needed. We use Dirac-type Slater orbitals, adapted from solutions to the hydrogen-like atom. This offers the perspective of performing relativistic calculations to the same accuracy as non-relativistic ones, with a comparable number of basis functions.

Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

The zeroth-order regular approximation (ZORA), a two component approximation to the Dirac equation that was earlier formulated and tested within the framework of density functional theory, is generalized to a treatment based on the Dirac-Fock equation. The performance of the ZORA equation and an improvement known as scaled ZORA is investigated, in particular with respect to orbital energies and various radial expectation values in the case of the xenon and radon atoms. The results of the simple ZORA approximation are shown to be quite close to the full Dirac-Fock method, except in the deep core region where the scaled version of the method is needed. It is found that a further approximation in which the density is calculated from the two-component ZORA orbitals alone gives satisfactory results, which is an important result from a practical point of view since in this way one can avoid calculating any two-electron integrals involving small-component basis functions.

No abstract available

We study the exchange interaction in the quasi-2D antiferromagnetic high T_{c} superconductor parent compound La_{2}CuO_{4} by all-electron, embedded cluster methods. Our material model is the cluster Cu_{2}O_{11}, embedded in a matrix of point charges. The SCF ground state configuration is characterized by Cu^{2+} and O^{2-} with the d-hole oriented in the [CuO_{2}]^{2-} plane along the Cu-O bonds. We admix to this an excited SCF configuration that has one Cu-hole transferred to the central O into the ground state configuration by nonorthogonal CI, while paying special attention to the variational balance of the singlet and triplet states. This results in a value for the exchange parameter of J = -120 meV, in good agreement with the experimental value of J = -128 meV.

The dielectric constant of a material is a macroscopic property that measures the reduction of the electrostatic forces between charged plates seperated by the material, compared to a vacuum as intermediate material. It is next encountered as a scaling parameter in Coulomb's law for interacting charges, not only in the force, but also in the energy. In deriving the theory for dielectrics, the macroscopic nature is essential: Only then is the basic assumption that the dielectric material is homogeneous and isotropic a valid one. The appearance of the dielectric constant as a simple scaling factor in Coulomb's law has tempted many computational chemists to forget about the macroscopic nature of the dielectric and to apply the screened Coulomb's law between charges, supposedly in a low-dielectric medium such as proteins, in microscopic force fields. Optimization of force fields even led to distance-dependent dielectric constants. Another use of the dielectric constant appears in the dielectric continuum reaction field approaches for the computations of solvation energies and solvent effects. The solute is embedded in a cavity surrounded by the dielectric. Specific interactions between solvent molecules and solute are thus neglected. The cavity size and dielectric constants of interior and exterior are optimized for the model. The aim of this article is to show, by calculations on interacting point charges embedded in cavities surrounded by dielectrics and microscopic models of low-dielectric materials by explicit polarizabilities, that are far as the dielectric constant is concerned anything can happen, depending on the nature of the charges, the distance to the cavity boundary, the spatial arrangement of charges, and polarizabilities. Thus, a warning is issued to injudicious use of dielectric models in microscopic calculations.

The effects of atomic and molecular electron correlation and Gaunt interaction on the transition energies between the ^{5}D_{0} and ^{7}F_{1,2} levels, arising from the 4f^{6} state of the europium ion, have been studied for the ion and for the [EuO_{6}]^{9-} cluster as it is found in the Ba_{2}GdNbO_{6} crystal. The calcnulations were performed using the MOLFDIR program package. The results are compared to work previously done by Visser et al. in which correlation effects has not been taken into account. The effect of the Gaunt interaction in found to be small. An important contribution to the correlation energy emerges from the configuration represented by the double excitations from 4d to 4f. This is in agreement with work performed by Jankowski and Sokolowski on Pr^{3+}. The results are now in better agreement with experiment but are still not satisfactory.

Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

The time-consuming step in coupled-cluster-Green-function or equivalently equation-of-motion-coupled-cluster calculation of ionization potentials for molecules is the solution of the CCSD equations. We investigate here the accuracy that can be obtained if the CCSD coefficients are replaced by their second-order-MBPT (MBPT(2)) analogs. We discuss some additional diagonal approximations that might prove especially useful in polymer calculations, and compare with traditional Green-function calculations based on a second-order approximation to the irreducible self-energy.

By combining the ideas of the direct perturbation theory approach to the solution of the Dirac equation with those underlying the regular expansion as used to obtain the two-component Chang-Pelissier-Durand Hamiltonian, a four-component form of the regular expansion is proposed. This formulation lends itself naturally to systematic improvement by a nonsingular form of perturbation theory. Alternatively it can be viewed as a double perturbation version of direct perturbation theory, where relativistic effects on the Hamiltonian and the metric are considered seperately and the Hamiltonian perturbation is summed to infinite order. The scaling procedure that was earlier shown to be exact in the case of a hydrogenic potential and that greatly improved the core orbital energies, is found to follow naturally from the current formulation. The accuracy of the various approximations to the wave functions is assessed with respect to several radial expectation values weighing different regions in the uranium atom as a test case.

We have investigated the performance of a fully close-coupled wave packet method and its symmetry-adapted version for a model problem of H_{2} scattering from LiF(001). The computational cost of the fully close-coupled methods scales linearly with the number of rotation-diffraction states present in the basis set, provided that the sparseness of the potential coupling matrix is taken into account. For normal incidence, the symmetry adapted version is faster than the conventional close-coupling wave packet method by almost an order of magnitude. An extension of the method to more realistic molecule-surface problems is considered.

The H_{2} + LiF(001) system was used to investigate the performance of the hybrid close-coupling wave packet (CCWP) method and of a symmetry adapted, fully close-coupled wave packet (SAWP) method for a molecule-surface problem characterized by fairly high corrugation. In the calculations, a realistic, phi-dependent model potential was used. The calculations were performed for a collision energy of 0.2 eV, with H_{2} initially in its j = 0 rotational state at normal incidence to the surface. Large increases in the computational efficiencies of both wave packet methods were achieved by taking advantage of the potential coupling matrices associated with both methods becoming sparser with increasing molecule-surface distance. For the present model problem and employing this increased sparseness at longer range, the SAWP method is faster than the CCWP method by a factor of 2. The potential usefulness of the SAWP method for dissociative chemisorption problems is discussed.

A method for calculating frequency-dependent polarizabilities and van der Waals dispersion coefficients, which scales favorably with the number of electrons, has been implemented in the Amsterdam Density Functional package. Time-dependent Density Functional Theory is used within the Adiabatic Local Density Approximation (ALDA). Contrary to earlier studies with this approximation, our implementation applies to arbitrary closed-shell molecular systems. Our results for the isotropic part of the van der Waals dispersion energy are of comparable quality as those obtained in TDCHF calculations. The ALDA results for the relative anisotropy of the dipole dispersion energy compare favorably to TDCHF and MBPT results. Two semi-empirical ways to calculate the dispersion energy anisotropy are evaluated. Large bases which include diffuse functions are necessary for a good description of the frequency-dependent properties considered here.

A review with 89 refserences. The embedding of a quantum mechanically described subsystem by classical representations of its surroundings is reviewed. The choices for a distributed monopole representation and a distributed (group) polarizability representation, as well as the continuum approach to model bulk effects, are discussed. Focus is on the practical implementation of the classical description in quantum chemical codes (in particular, HONDO 8.1). Expressions are given for the self-consistent coupling between the classical partitions (dipole polarizabilities and boundary surface dipoles and charges) and for the coupling between classical and quantum partitions. The latter is mediated through expanded, rather than exact, potentials and fields. In this way, the computation of only a limited number of formal interactions between unit charge distributions located at the expansion centers suffices to evaluate the reaction field contributions. The electronic part of the coupling can be included in the Hamiltonian via the Fock matrix. The field operators, as well as the one- and two-electron matrix elements over the basis functions, are simple. The expressions for these are given explicitly. Nonequilibrium potentials and Monte Carlo sampling over classical degrees of freedom have been added to better mimic experimental conditions.

Inelastic, state-to-state parity-resolved, relative cross sections for the NO(^{2}P_{1/2},J=1/2^{-}) + Ar to NO(^{2}P_{1/2},J'P') + Ar collision have been obtained in a crossed beam experiment. Initial state preparation of NO molecules is realized with 99% purity via the hexapole state selection technique. The parity-resolved rotational state distribution of NO product molecules is obtained by means of laser-induced fluorescence (LIF) spectroscopy on the g(0,0)-band. The experimental cross sections are in agreement with those obtained previously in a comparable study, but some discrepancies remain with results from close-coupling scattering calculations.

The Dirac-Fock-CI method is reviewed. The MOLFDIR program package that was developed for calculations of this type on molecular systems is described in detail. Computational details of some recent applications are presented to give an impression of the computational resources necessary.

No abstract available

In this paper we will address the question of how to obtain energies from functionals when only the functional derivative is given. It is shown that one can obtain explicit expressions for the exchange-correlation energy from approximate exchange-correlation potentials using line integrals along paths within the space of densities. The path dependence of the results is discussed and criteria for path independence are given. Derivations are given of upper and lower bounds to the exchange-correlation energy in terms of the exchange-correlation potential at the beginning and the end point of a certain path. We further express the kinetic part T_{xc} of the exchange-correlation energy in terms of a line integral and derive a constraint on approximate correlation potentials. We show how to use the line-integral formalism to derive the requirements that exchange-correlation potentials must fulfill in order to make the exchange-correlation functional satisfy some symmetry property such as rotationaland translational invariance and scaling properties. Finally, we will discuss the use of line integrals along a path in density space to obtain energy differences, notably, the bonding energies of molecules, from exchange-correlation potentials. These last results generalize the transition-state formulations of Slater and Ziegler.

A scheme of approximation of the Kohn-Sham exchange potential v_{x} is proposed, making use of a partitioning of v_{x} into the long-range Slater v_{S} and the short-range response v_{resp} components. A model potential v_{resp}^{mod} has been derived from dimensional arguments. It possesses the proper short-range behavior and the atomic-shell stepped structure characteristic for v_{resp}. When combined with the accurate v_{S}, v_{resp}^{mod} provides an excellent approximation to the exchange potential of the optimized potential model v_{x}^{OPM}. With the generalized-gradient approximation to v_{S} vrespmod provides an efficient density-functional-theory approach that fits closely the form of the accurate exchange potential and yields reasonably accurate exchange and total energies as well as the energy of the highest occupied orbital.

The molecular Kohn-Sham (KS) exchange-correlation potential vxc has been constructed for LiH from the correlated ab initio density rho by means of the simple iterative procedure developed by vanLeeuwen and Baerends [Phys. Rev. A 49, 2421 (1994)]. The corresponding KS energy characteristics, such as the kinetic energy of noninteracting particles T_{s}, kinetic part of the exchange-correlation energy T_{c}, and energy of the highest occupied molecular orbital epsilon N, have been obtained with reasonable accuracy. A relation between the form of v_{xc} and the electronic structure of LiH has been discussed. Test calculations for the two-electron H_{2} molecule have shown the efficiency of the procedure.

The recently proposed stochastic diagonalization method is applied to the ab initio quantum chemistry CI problem. In this context it can be viewed as a multi-reference CI method with dynamic selection of important configurations. The method is compared with other methods and tested by calculations on a number of small molecular systems for which accurate results are available. A calculation on the Cr_{2} dimer is presented to show the capability of the algorithm to find short expansions of molecular wavefunctions.

It is shown how by combining the ideas of the direct perturbation theory approach to the solution of the Dirac equation and the regular expansion as used in the Chang-Pelissier-Durand Hamiltonian one can derive a spin-free approximation to the Dirac equation that resembles a similar equation recently derived by K. Dyall (1994). However, unlike that equation, the present approach is entirely free of singular operators even in the case of a potential containing an attractive Coulomb singularity such as encountered in atomic and molecular Dirac-Fock theory.

The Dirac-Fock-CI method is reviewed with 55 references. The MOLFDIR program package that was developed for calculations of this type on molecular systems is described in detail. Computational details of some recent applications are presented to give an impression of the computational resources necessary.

It is shown how the regularized two-component relativistic Hamiltonians of Heully et al. and Chang, Pelissier, and Durand can be viewed as arising from a perturbation expansion that unlike the Pauli expansion remains regular even for singular attractive Coulomb potentials. The performance of these approximate Hamiltonians is tested in the framework of the local density approximation and the relation of their spectrum to that of the Dirac Hamiltonian is discussed. The circumstances under which the current approximations are superior to the Pauli Hamiltonian are analyzed. Finally, it shown how the Hamiltonians could be used within the context of conventional Hartree-Fock theory.

No abstract available

Molecules and crystals that contain heavy elements, in particular 5d transition metals and rare earths, are of growing interest in chemistry and physics. Whereas spectra and other properties of light atoms and molecules can be satisfactorily explained starting from a non-relativistic quantum mechanical model, one has to consider the theory of relativity more thoroughly if one wants to describe the electronic structure of elements that belong to the lower regions of the periodic table. The influence of relativity is usually accounted for by introducing additional operators (spin-orbit coupling, mass-velocity correction, Darwin term) to the nonrelativistic Schrödinger equation. An alternative, more fundamental, approach starts from the Dirac-Coulomb-(Breit) equation which is the approximately relativistic many-electron form of Dirac's one-electron equation. In this article the Dirac-Fock-CI approach that was derived at the University of Groningen to handle this equation will be reviewed. Besides technical details of the method, like the use of kinetically and atomically balanced basis sets, the definition of the CI space and the use of double group symmetry, some attention will be paid to the fundamental aspects and interpretation of the Dirac-Coulomb-(Breit) equation. Results of calculations on the PtH molecule and on the transition-metal complexes XF_{6}^{2-} (X = Co, Rh, Ir) will serve to illustrate the method and to give an impression of the type of results that may be obtained.

In this paper an overview is given of MOLFDIR, the Molecular Dirac-Fock-CI Program Package, developed at the University of Groningen. The structure and the possibilities of the MOLFDIR Package are shown together with some technical aspects concerning the implementation of the four-component Dirac-Fock-Breit and RASCI equations. Also a summary is given of the relativistic ab initio calculations that have been performed using this program package.

A review with 25 references on the program suite MOLFDIR which allows to carry out fully relativistic, all electron ab initio calculations for polyatomic molecules, based on the Dirac-Coulomb hamiltonian. The program handles open shells by defining a suitable averaged Dirac-Fock operator. Gaussian basis sets in conjunction with kinetic and atomic balance relations are employed to expand the large and small components of the molecular Dirac spinors. The CI capabilities include a complete CI in the manifold of open shell states and a multi-reference RAS-CI carried out in the space of the occupied and virtual poitive energy Dirac-Fock solutions.

Apart from relativistic effects originating from high kinetic energy of an electron in a flat potential, which are treated in first order by the Pauli Hamiltonian, there are relativistic effects even for low-energy electron if they move in a strong Coulomb potential. The latter effects can be accurately treated already in the zeroth order of an expansion of the Foldy-Wouthuysen transformation, if the expansion is carefully chosen to be nondivergent for r goes to 0 even for Coulomb potentials, as shown by Van Lenthe et al. [J. Chem. Phys. 99, 4597 (1993)] (cf. also Heully et al. [J. Phys. B 19, 2799 (1986)] and Chang et al. [Phys. Scr. 34, 394 (1986)]). In the present paper, it is shown that the solutions of the zeroth order of this two-component regular approximation (ZORA) equation for hydrogen-like atoms are simply scaled solutions of the large component of the Dirac wave function for this problem. The eigenvalues are related in a similar way. As a consequence, it is proven that under some restrictions, the ZORA Hamiltonian is bound from below for Coulomb-like potentials. Also, an exact result for the first order regular approximate Hamiltonian is given. The method can also be used to obtain exact results for regular approximations of scalar relativistic equations, like the Klein-Gordon equation. The balance between relativistic effects originating from the Coulombic singularity in the potential (typically core penetrating s and p valence electrons in atoms and molecules) and from high kinetic energy (important for high-energy electrons in a flat potential and also for core-avoiding high angular momentum (d, f, and g states in atoms)) are discussed.

The Kohn-Sham potential v_{s} of an N-electron system and the potential v_{eff} of the Euler-Lagrange equation for the square root of the electron density are expressed as the sum of the external potential plus potentials related to the electronic structure, such as the potential of the electron Coulomb repulsion, including the Hartree potential and the screening due to exchange and correlation, a potential representing the effect of Fermi-Dirac statistics and Coulomb correlation on the kinetic functional, and additional potentials representing ``response'' effects on these potentials. For atoms several of these potentials have distinct atomic shell structure: One of them has peaks between the shells, while two others are step functions. In one of those step functions the steps represent characteristic shell energies. Examples of the potentials extracted from the optimized potential model (OPM) are presented for Kr and Cd. Correlation potentials, obtained by subtracting the exchange potential of the OPM from (nearly) exact Kohn-Sham potentials, are discussed for Be and Ne.

In this paper we will discuss relativistic total energies using the zeroth order regular approximation (ZORA). A simple scaling of the ZORA one-electron Hamiltonian is shown to yield energies for the hydrogenlike atom that are exactly equal to the Dirac energies. The regular approximation is not gauge invariant in each order, but the scaled ZORA energy can be shown to be exactly gauge invariant for hydrogenic ions. It is practically gauge invariant for many-electron systems and proves superior to the (unscaled) first order regular approximation for atomic ionization energies. The regular approximation, if scaled, can therefore be applied already in zeroth order to molecular bond energies. Scalar relativistic density functional all-electron and frozen core calculations on diatomics, consisting of copper, silver, and gold and their hydrides are presented. We used exchange-correlation energy functionals commonly used in nonrelativistic calculations; both in the local-density approximation (LDA) and including density-gradient ("nonlocal") corrections (NLDA). At the NLDA level the calculated dissociation energies are all within 0.2 eV from experiment, with an average of 0.1 eV. All-electron calculations for Au_{2} and AuH gave results within 0.05 eV of the frozen core calculations Ag_{2} and AgCu and CuH.

The lowest excited states of a Cu^{+} impurity in NaF are studied with Hartree-Fock-Roothaan calculations on large embedded clusters. Attention is focussed on the question of how localized these excitations are. The interest in the nature of the excited states has recently revived, because it was concluded from OD-EPR experiments on the luminescent state of Cu^{+} in NaF, that there is almost no electron spin density in Cu4s. However, 1- and 2-photon absorption spectra on the Cu^{+} impurity in NaF were interpreted in terms of localized Cu^{+}(3d^{10}) to Cu^{+}(3d^{9}4s^{1}) excitations. Probably low lying states exist, in which the excited electron and the electron hole are only loosely coupled.

Drie eeuwen geleden beschreef Christiaan Huygens licht als een golfverschijnsel. Isaac Newton zag licht daarentegen als een deeltjesverschijnsel. De ontwikkeling van de quantumtheorie overbrugde de tegenstelling tussen de golftheorie en de deeltjestheorie. Dankzij ons huidige begrip van licht beschikken we over tal van methoden waarbij licht ons informatie over de wereld om ons heen verschaft. In dit artikel gaat de auteur in op de wisselwerking tussen licht en materie.

In LICHT OP MATERIE I beschrijft de auteur hoe de golflengte waarbij molekulen licht absorberen, verstrooien en uitzenden wordt bepaald door de vibratie-, rotatie- en elektronenniveaus in een molekuul. Maar de mate waarin dat gebeurt, is eveneens van wezenlijk belang voor de toepassing van de spectroscopie als informatiebron. Responsfuncties geven aan hoe molekulen zich bij de wisselwerking gedragen. Diverse spectroscopische meetmethoden laten zich dankzij deze responsfuncties beter begrijpen.

The Direct Reaction Field (DRF) method aims at embedding a quantum mechanical system in a large semi-classical environment. In its present form, the classical part of the system is modeled by point charges, interacting poilarizabilities amd/or a dielectric contiuum which may have finite ionic strength. In the DRF approach typically all interactions are made part of the Hamiltonian for the quantum part H=H_{0}+H_{STAT}+H_{RF} where H_{0} is the usual moleculat Hamiltonian of the quantum mechanical part, H_{STAT} the static potential, and H_{RF} the reaction potential, due to the classical parts.

Assuming linear response of the classical part, we first solve the Poisson (or in case of finite ionic strength for the continuum: the linearized Poisson-Boltzmann) equation by a Boundary Element Method for the interacting polarizabilities in a cavity of arbitrary size and form.

The solution is formally obtained in the form of a 'relay matrix' from which integrals over any basis set can be evaluated and which are - together with those for the static potential - added to the corresponding integral of H_{0}. After that, a 'normal' quantum mechanical calculation of any desired type or accuracy gives the interaction energy as the difference of expectation values DU=<Y|H|Y>-<Y|H_{0}|Y>.

Thus we may obtain first and second order environmental effects on the wave function and properties of the quantum part.

In principle, all parameters for the classical part may be obtained from appropriate ab inition calculations on the constituting subsystems, which makes this approach an example of a 'global' simulation. The method depends critically on the extent to which the various parts of the system are 'separated', i.e. are '(not) overlapping'. Here we will discuss some more or less successful applications, and pint at some fundamental problems concerning the mixing of quantum mechanical and classical descriptions.

In this work we analyze the properties of the exchange-correlation potential in the Kohn-Sham form of density-functional theory, which leads to requirements for approximate potentials. Fulfilment of these requirements is checked for existing gradient-corrected potentials. In order to examine the behavior of approximate potentials over all space we compare these potentials with exact Kohn-Sham potentials calculated from correlated densities using a newly developed iterative procedure. The main failures in the existing gradient-corrected potentials arise in the asymptotic region of the atom where these potentials decay too fast and at the atomic nucleus where the potentials exhibit a Coulomb-like singular behavior. We show that the main errors can be corrected by a simple potential in terms of the density and its gradients leading to considerably improved one-electron energies compared to the local-density approximation. For Be and Ne it is shown that the electron density is improved in the outer region.

No abstract available

The electronic spectra of the transition metal complexes CoF_{6}^{2-}, RhF_{6}^{2-} and IrF_{6}^{2-} that occur in the solids Cs_{2}MeF_{6} are calculated. Hartree-Fock and Dirac-Fock calculations followed by non-relativistic and relativistic CI calculations resp. are used to study the influence of relativity and electron correlation. The calculated transitions are found to agree fairly well with experiment, the largest discrepancies arising from the neglect of differential dynamic electron correlation effects.

No abstract available

No abstract available

Consider two orbital sets c_{k}, k = 1...m and h_{l}, 1 = 1...n, which are mutually nonorthogonal. Provided that n > m, at least n - m orbitals of the set {h}. The orthogonalization of the remaining orbitals of {h} to the set {c} requires a transformation in which the c_{k} appear explicitly. The orthogonalization of one orbital set to another is relevant for SCF optimizations in a truncated basis set, in the presence of frozen occupied orbitals. Examples are frozen core calculations, ECP calculations, and embedded cluster calculations, where the cluster is embedded in a frozen environment. A simple orthogonalization scheme, which makes use of a corresponding orbital transformation, is presented. It is suggested that with a small, well-defined extension of the set {h_{l}} the complete orthogonalization can be done with a transformation in which the {c} do not appear explicitly.

The response approach to couple cluster (CC) expectation values devised by Monkhorst and the normal coupled cluster method (NCCM) discussed by Arponen and Bishop are considered from a diagrammatic point of view. The perturbative diagrammatic content of the operator that parametrizes the NCCM bra state is discussed in detail. The method is applied to the calculation of the one-particle reduced density matrix of the Be atom and the HF molecule for different values of the internuclear distance. Various contributions to the total energy obtained in the NCCM framework are compared to results from accurate multireference CI calculations. Such a comparison provides a much more stringent test on the accuracy of the CC formalism than will a comparison of total energy alone.

Detailed working equations are derived for the ionization part of the single-particle Green's function within the coupled cluster Green's function (CCGF) framework. The CCGF method is applied to the calculation of vertical ionization potentials (IPs) of a number of small molecules, notably, HF, N_{2}, CO, F_{2}, CS, C_{2}H_{4}, H_{2}O, and H_{2}CO. The results for the outer-valence IPs, with an average error of 0.12 eV, compare favorably to third-order equation-of-motion calculations within the same basis set (average error 0.28 eV) and outer-valence GF (OVGF) values taken from the literature (average error 0.17 eV). Ground-state properties that derive from the CCGF are compared to expectation values obtained in the related normal coupled cluster methods (NCCM) approximation from a formal point of view. Correlation energies obtained in CCGF are compared to CCSD results for the above series of molecules and, in addition, the so-called true correlation energy density as obtained from the CCGF is compared to the result from an accurate MR-CI calculation for a highly correlated system: the HF molecule at large internuclear separation.

Recently the authors presented an extension of the direct reaction field (DRF) method, in which a quantum system and a set of point charges and interacting polarizabilities are embedded in a continuum that is characterized by a dielectric constant e and a finite ionic strength. The reaction field of the continuum is found by solving the (linearized) Poisson-Boltzmann equation by a boundary element method for the complete charge distribution in a cavity of arbitrary size and form. Like many other authors, the continuum contribution to the solvation energy decreases rapidly with the relative cavity size. The literature gives no clues for the definition of the cavity size beyond physical intuition or implicit fitting to experiment or otherwise desired results. From theoretical considerations, a number of limitations on the position of the boundary are derived. With a boundary defined within these limitations, the experimental hydration energies cannot be reproduced, mainly because of the neglected specific interactions. The description of the solute's electronic states also depends on the solvation model. Probably one or more explicit solvent layers are needed to obtain reliable solvation and excitation energies.

The CF_{3}I(5pp-6s) Rydberg transitions in the energy range 56,700-64,000 cm^{-1} are investigated using (2 + 1) resonance-enhanced multiphoton ionization. The polarization of the two-photon transitions is used to definitely assign the symmetries of the resonant intermediate states. The four allowed electronic transitions in the (5pp-6s) manifold were assigned and some vibrational constants in the excited states were determined. Hot band spectra were obtained in a supersonic expansion of CF_{3}I through an oven. The upper spin-orbit components (the ^{2}E_{1/2} ion core states) are perturbed by a dissociative state at approximately 63,000 cm^{-1}, possibly the s-s* transition centered on the C-I bond. Density functional calculations were performed in order to help determine the nature of the perturbing states. Vibronic interactions in the excited states are investigated, and evidence is seen for quadratic Jahn-Teller interactions for n_{6} in the lower (^{2}E_{3/2}) spin-orbit state.

In the present work, potential-dependent transformations were used to transform the four-component Dirac Hamiltonian into relativistic, effective, two-component, regular Hamiltonians. To zeroth order, the expansions give second-order differential equations (just like the Schrödinger equation), which already contain the most important relativistic effects, including spin-orbit coupling. One of the zeroth- order Hamiltonians is identical to the one obtained earlier by Ch. Chang, et al., (1986). By using these Hamiltonians, self-consistent all-electron and frozen-core calculations, as well as first-order perturbation calculations were done for the uranium atom. They gave very accurate results, especially for the one-electron energies and electron densities of the valence orbitals.

Fully relativistic all-electron SCF calculations based on the Dirac-Coulomb Hamiltonian have been performed on the three lowest lying states of the PtH molecule. The resulting four-component Dirac-Hartree-Fock (DHF) molecular spinors are subsequently used in relativistic CI (CI) calculations on the five lower states of PtH. Spectroscopic properties are obtained by fitting the potential curve to a Morse function and show good agreement with experimental data. The effect of relativistic corrections to the Coulomb electron-electron interaction is investigated at the DHF level and is found to be insignificant for the molecular spectroscopic properties investigated by the authors. The CI wave functions are found to have only one dominant configuration, indicating a lack of static correlation. Dynamic correlation in the d shell is, however, important for the spectroscopic properties of PtH. The results conform with a bonding scheme in which the three lower and two upper states of PtH are assigned 5d_{3/2}^{4} 5d_{5/2}^{5} s_{1/2}^{2} electronic configurations, resp. The configurations are only approximate and are perturbed by 5d participation in bonding. The stability of the Pt-H bond is explained in terms of the relativistic stabilization of the 6s orbital in analogy with the electron affinity of the platinum atom.

The tautomer equilibrium of a number of 4-substituted imidazoles in the gas phase and in aqueous solution was calculated by combining quantum chemical ab initio calculations on the tautomers in the gas phase with classical electrostatics calculations (a continuum model and a Monte Carlo method with a limited number of discrete solvent molecules) to evaluate the solvation energy. The results were in good agreement with experimental data from ^{15}N-NMR studies. It was found to be important to include counter ions in the calculations for imidazoles with a charge side chain. The methods for evaluation of the solvation energy were compared.

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A review with 13 references is given with discussion of cluster models for polar transition metal complexes. A recurrent question in the discussion of electronic properties of solids is wether a theoretical treatment can best be based on a localized (cluster) model or on a delocalized (band) model. This is independent of one's choice for a wave functional or a density functional form of theory. The question will be discussed in terms of the ionic model for solids taking the insulator CuCl as an example. It will be shown that a local approach can become essential as a starting point in the treatment of excited and ionized states when local many-electron effects including multiplet coupling and electronic relaxation are larger than the intersite interactions that determine the bandwidth. This is illustrated by some recent results of hole state calculations on Cu^{2+} as a function of internuclear distance. Recent results of cluster calculations on Cu(2p) and O(1s) hole states in La_{2}CuO_{4} are shown to confirm current experimental assignments.

The embedded cluster approximation is essential for describing local excitations in ionic transition metal compds. In these compounds, ionizations and local excitations are accompanied by large electronic relaxation effects. These can efficiently be described in the cluster model. It is argued that the importance of orbital relaxation naturally leads to a nonorthogonal CI treatment of such excitations. In a nonorthogonal CI wave function each configuration state function is expressed in terms of one-electron orbitals that have been optimized for that configuration. The conditions under which this CI method can be applied are discussed using results on 3d hole states of Cu^{2+} as an example. The method is illustrated with some recent results of calculations on hole states in La_{2}CuO_{4}.

The single-particle Green's function is used to generate a new zero-order Hamiltonian. The idea to generate a new zero order from the previous zero order by incorporating perturbative corrections up to certain order is attractive since it allows an iterative procedure to repeatedly improve the results by decreasing the perturbation. In particular, in those cases where the Hartree-Fock Hamiltonian is not a good approximation to the full Hamiltonian and where perturbation theory usually does not produce sufficiently accurate results, one might hope that such a repetitive procedure ultimately yields an improved zero order and accurate perturbative corrections from this newly generated zero order. Two such approaches are investigated: first, one in which the w-independent part of the self-energy is fully incorporated in the zero order and, second, one in which the correlation energy is incorporated in a one-electron potential in an average way. Numerical calculations are reported.

Diagrammatic and Coupled Cluster techniques are used to develop an approach to the single-particle Green's function G which concentrates on G directly rather than first approximating the irreducible self-energy and then solving Dyson's equation. As a consequence the ionization and attachment parts of the Green's function satisfy completely decoupled sets of equations. The proposed Coupled Cluster Green's Function method (CCGF) is intimately connected to both Coupled Cluster Linear Response Theory (CCLRT) and the Normal Coupled Cluster Method (NCCM). These relations are discussed in detail.

Results are presented of all-electron molecular relativistic (Hartee-Fock-Dirac) and nonrelativistic (Hartree-Fock) calculations followed by a complete open shell CI (COSCI) calculation on an EuO_{6}^{9-} cluster in a Ba_{2}GdNbO_{6} crystal. The results include the calculated energies of a number of states derived from the f^{6}-manifold and ^{5}D-^{7}F luminescence transition wavelengths. The calculations were performed using the molecular Fock-Dirac (MOLFDIR) program package developed in our laboratory. The theory and methods employed in this package are briefly described. The physical models used to analyze the Eu^{3+} impurity states range from a bare Eu^{3+} ion to an EuO_{6}^{9-} cluster embedded in a Madelung potential representing the rest of the crystal. It is necessary to use the embedded cluster model to get a reasonable description of the crystal field splittings of the states arising from the f^{6}-manifold. The results indicate that the calculated splittings are very sensitive to the orbitals used. It is therefore essential that relativistic orbitals be used from the outset.

Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, IL 62901, USA,

Organic and Molecular Inorganic Chemistry, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Ab initio molecular orbital methods are used to study local anesthetics of the ester, analide and carbamate types. Electrostatic molecular potential contour maps were evaluated for aromatic parts of those type of drugs.

Because the main constituents of cell membranes are lipids and proteins, our further SCF MO studies concerned interaction of polar groups of local anesthetics with possible associative sites of nerve membrane. Aniline, formanilide and trimethylamine represent possible associative sites of the local anesthetics procaine and lidocaine. Dimethylphosphate mono anion, o-phosphate monoanion, formate anion and acetamide represent the associate sites of the membrane. Finally, the binding of the amine group of drug with the Na^{+}, K^{+}, Ca^{2+} and Cl^{-} ions present *in vivo* is also investigated.

The embedded cluster approximation is essential for describing local excitations in ionic transition metal compds. In these compounds, ionizations and local excitations are accompanied by large electronic relaxation effects. These can efficiently be described in the cluster model. It is argued that the importance of orbital relaxation naturally leads to a nonorthogonal CI treatment of such excitations. In a nonorthogonal CI wave function each configuration state function is expressed in terms of one-electron orbitals that have been optimized for that configuration. The conditions under which this CI method can be applied are discussed using results on 3d hole states of Cu^{2+} as an example. The method is illustrated with some recent results of calculations on hole states in La_{2}CuO_{4}.

The direct reaction field (DRF) approach is a practical method for incorporating environmental (e.g. solvation) effects on a system in which the electronic charge distribution is described by wave functions and the "solvent" is modeled by a collection of interacting point charges and (point) polarizabilities. The DRF method is briefly summarized. A numerical solution is presented (based on a boundary element method) of the Poisson (-Boltzmann) equation for a set of quantum mechanical and/or point charges in a cavity in a dielectric continuum which may have a finite ionic strength. The solution of these equations is described for the situation in which the cavity contains a set of charges with polarizabilities.

Results and details of molecular Fock-Dirac-(Breit) calculations on CH_{4}, SiH_{4}, GeH_{4}, SnH_{4}, and PbH_{4} obtained with the MOLFDIR program package are presented and compared with other calculations and experimental results. The relativistic ground state energies (including the Breit interaction) of the atoms C, Si, Ge, Sn, and Pb, necessary for reference purposes, have been calculated using a small relativistic CI. For the heavier systems perturbation theory over-estimates the relativistic bond length contraction. The Breit interaction has only a small effect on the bond lengths.

No abstract available

General contraction is a well known means to reduce the computational effort for calculating electronic wave functions using basis set expansion techniques. In contrast to fixed contraction, optimal flexibility is available in constructing the "best" basis functions from basis set primitives. For relativistic four-component Dirac-Hartree-Fock calculations (and beyond), the variational space may consist of separate basis functions for the "Large" and "Small" component bases. The choice, and therefore also the contraction, of the Small component basis relative to the Large component basis is non-trivial. In particular in the case that "Kinetic Balance" is used to define the Small component basis relative to the Large component basis contraction imposes a severe problem, because the better the contraction of the Large component, the worse the kinetically balanced Small component counterpart becomes. Solutions to the dilemma are provided and have been implemented. Figures on reduction in the computational effort are given.

An open shell version has been developed for the Molecular Hartree-Fock-Dirac program package MOLFDIR. This program, originally developed as a closed shell program in 1985, can be used to calculate all-electron four-component Hartree-Fock-Dirac Self Consistent Field solutions for -in principle- molecules of general shape and uses separate gaussian basis sets to expand the large and small component functions. In practice basis sets are chosen to be kinetically balanced; technically, the program is not restricted to such basis sets. Implementation, advantages and limitations due to the Dacre and Elder algorithm for integral evaluation and Fock matrix set-up are discussed and results of test calculations are presented.

The effects of the solvent on the stability of the zwitterion in the active site of papain is investigated with numerical methods. The solvent is represented by a homogeneous dielectric continuum surrounding a cavity, defined by a fragment of the protein enclosed by a surface obtained following Connolly's method. The discretisized boundary surface is used to solve the Poisson equation in its integral form by means of a numerical approximation based on the boundary element method (BEM), resulting in a set of surface polarization charges. The solvent effect on the proton transfer in papain is studied on the basis of MO-SCF-direct reaction field (DRF) calculations of the energy and charge distribution of the fragment in the field of the surface charges. The role of Asp-158 in the proton transfer in the active site of papain is reevaluated in the presence of the solvent. It is concluded that the effect of the negative charge of Asp-158 is nearly completely screened by the solvent.

A demonstration of kinetic balance failure in heavily contracted basis sets is given. Other possible methods of constructing small component basis sets for 4-component relativistic calculations are discussed. The position of the additional negative energy levels in extended balance calculations in some recent many-electron calculations is examined.

Relativistic calculations on UO_{2} show that relativity leads to substantial bond lengthening in this compound, in contrast to the bond contraction found almost exclusively for other compounds. The bond lengthening is not caused by the relativistic expansion of the 5f valence AO of U, which is the primary bond forming orbital on U in UO_{2}. The origin of the bond lengthening can be traced back to the semi-core resp. subvalence character of the U 6p AO. The valence character of 6p shows up in an increasing depopulation of the 6p upon bond shortening, and hence loss of mass-velocity stabilization. The core character of 6p shows up in large off-diagonal mass-velocity stabilization. The core character of 6p shows up in large off-diagonal mass-velocity matrix elements <5p|h_{MV}|6p> which are shown to have an overall bond lengthening effect. The larger expansion in UO_{2} than in UO_{2}^{2+} is due to destabilization of U levels in UO_{2}, caused by repulsion of the two additional 5f electrons.

No abstract available

A review with 19 references. The direct reaction field (DRF) method, developed to incorporate the effects of a (large) semiclassical environment into the Hamiltonian of a quantum mechanical system, is briefly reviewed. It is shown that the DRF method behaves, at least, like a supermolecular SCF calculation. With the water dimer as an example, the similarity with the SCF procedure is demonstrated. An application to the interaction between the active site of papain and the remaining 3000 or so atoms of this protein shows the inadequacy of dielectric constant models and the necessity of including atomic polarizabilities in model force fields.

The well-known expression for the total energy in terms of the single-particle many-body Green's function is analyzed in detail. In particular the relation between the n-th order Møller-Plesset energy and the energy calculated from a Green's function generated by the n-th order Møller-Plesset energy can be expressed in terms of the Green's function. The H_{2} molecule is studied in a minimal basis to serve as a model in which exact results can be easily obtained. Numerical calculations are performed for H_{2}, He, Be, LiH, Ne, HF, H_{2}O, NH_{3}, and CH_{4} and the results are analyzed in detail and compared with other calculations.

The electronic structure of [CuO_{2}]^{2-} sheets, as occur in La_{2}CuO_{4}, is studied by ab initio non-orthogonal configuration interaction (NOCI) calculations on the small copper-oxide clusters CuO_{4} and Cu_{2}O_{7}. In the NOCI approach the wave function is obtained from CI calculations between relaxed Hartree-Fock states. With this approach important local relaxation effects, as well as charge transfer interactions can be taken into account.

The cluster ground states can be characterized by Cu(d^{9}) O(p^{6}). States with one extrinsic hole have predominant Cu(d^{9}) O(p^{5}) character with admixture of Cu(d^{8}) O(p^{6}). This admixture is especially important for stes with the extrinsic hole mainly in O(ps).

The magnitude of the relativistic contraction or expansion of AOs is usually obtained by comparison of the expectation values of r in a Dirac-Fock calcn. and in a Hartree-Fock calculation. As, however, the Dirac Hamiltonian is implicitly given in a different picture from the non-relativistic Schrödinger Hamiltonian, the operator r does not correspond to the same physical quantity in the two cases. A proper definition of relativistic AO contraction/expansion should use the same physical quantity in both the relativistic and non-relativistic cases; for instance experiments with photons measure matrix elements of rcharge, which is represented by the operator r in the Dirac picture and by U_{FW}rU_{FW}^{†} in the Schroedinger picture (U_{FW} is the Foldy-Wouthuysen transformation). Accordingly, the conventional values of the relativistic AO contraction consists of two contributions. One is due to the relativistic modification of the orbital; the other one is due to the different meanings of r in the Schrödinger and Dirac pictures. This latter difference turns out to be significant for 1s AO, where it is 50%. The large relativistic contraction of valence s AO of heavy elements in investigated. Using perturbation theory or the resolution of the identity into projection operators, the orthogonality of the valence AO on the strongly contracted inner core orbitals is shown to have a slight valence-expanding effect, while mixing in of the higher continuum orbitals by the relativistic correction of the Hamiltonian is responsible for the overall contraction.

The deformation energetics of static polyethylene chain defects were determined by molecular mechanics and energy minimization. The defects considered were dispiration and dislocation (both interstitial- and vacancylike), disclination, chain twist boundary (90 degree and 180 degree), and partial dislocation boundary. For these defects, energy-minimized structures were calculated under action of tensile forces. One group of defects had moduli and deformation energetics identical with those of an all-trans chain, whereas the other group of defects had substantially lower moduli and a different deformation behavior. From the analysis of the distribution of deformation energy over the several degrees of freedom and by comparison to an all-trans chain, it could be concluded that defects inside a crystalline matrix are not weak links in polyethylene fibers. Weak links were chains having higher moduli than the surrounding matrix.

A review with 149 refserences of empirical potential models in relation to the theory of intermolecular interactions. It is argued that empirical force fields, and many ab initio approaches using supermolecules, neglect the problem of separating intermolecular interaction energies from the intramolecular energy change due to the presence of other particles. In this way, strong interdependencies between potential function parameters are introduced, hampering systematic improvement of these functions and transfer of parameters from, e.g., crystals to the liquid phase. Conspicuous differences between existing force fields are observed with respect to charge distributions, hydrogen bond strengths, and dispersion coefficients. The usual effective potential approach for induction and dielectric effects is invalidated by qualitative and quantitative different results obtained with more detailed models. Present day force fields appear to be least reliable in situations where the electric field plays a prominent role, such as binding sites, active centers, and ionic solutions - cases which are at the heart of many molecular engineering studies. Guidelines and practical tools for developing better force fields are provided by quantum mechanical perturbation theory. Molecular interactions can be expressed in terms of well-defined and calculable monomeric properties. An overview is given of approaches to evaluate these quantities and to represent them in many-body interaction calculations.

No abstract available

Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

A review with 21 references.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

A theoretical study of the influence of several elements of primary and secondary structure on the H^{+} transfer between cysteine-25 and histidine-159 in papain is reported. Two theoretical approaches, the direct reaction field method and the point charge approximation, are used in these calculations in order to evaluate the necessity of representing the protein environment as a polarizable medium.

Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

Density functional calculations on Pt(CN)_{4}^{2-} (I) and the recently reported Tl_{2}Pt(CN)_{4} (II) were carried out in both the nonrelativistic and quasirelativistic limits. This yielded a new and detailed understanding of the electronic structure of I. Electronic transition energies for I were calculated from state function energy differences to help clarify many of the spectroscopic properties of this ion. The interaction between I and Tl^{+} ions in II is largely ionic in nature but with a substantial covalent component (189 kJ mol^{-1}). The Tl^{+} ions in II provide a spectroscopic probe that could enable a rigorous comparison to be made between electronic structure calculations and polarized single-crystal absorption and emission spectra.

Ab initio charge distributions for amino acid dipeptides are derived utilizing 2 medium-sized basis sets. Peptide charges differ in two ways from those of existing force fields: the magnitude of the peptide dipole and the dependency on the residue type. The merging of charge distributions of side chain and backbone fragments within a semiclassical model including polarization is investigated. Polarization plays a small, but distinct role in improving the correspondence with ab initio data derived for the complete dipeptide. A description in terms of partly overlapping, interacting fragments correlates well with the ab initio data. The method can be used to derive the electrostatic properties of biological macromolecules by combining accurate descriptions of short range interactions (using good quality basis sets on not too small fragments) with good classical models of long range interactions (using multicenter multipole expansions and atomic polarizability tensors). Factors limiting the accuracy of the present representations are discussed.

The proton transfer between the cysteine (Cys)25 and histidine (His)159 residues in the active center of the proteolytic enzyme papain is investigated with the Hartree-Fock SCF direct reaction field method. The active center is treated quantum mechanically, while the environment is represented by interacting partial charges and polarizabilities. All protein atoms around the active site are included explicitly in the calculations. In this way a complete description is given of both the electrostatic and the dielectric properties of the enzyme. The protein matrix stabilizes the zwitterionic form of Cys-His, which is thought to be the catalytically active state much more than the neutral configuration. The most important contribution to the stabilization comes from the a-helix to which Cys25 is attached; more than half of its effect is due to the backbone atoms of Cys25 itself. Other important factors are the asparagine-175 side-chain and the solvent. Solvent effects are estimated by means of Monte Carlo calculations of crystal water molecules that are located near the active site. The total energies of the neutral and zwitterionic structures are similar, confirming the idea that a zwitterion can exist in the active center of papain. The energy difference, however, is sensitive to the geometry of the active site, suggesting that the 2 structures are in thermal equilibrium. Classical analogs of the quantum mechanical interaction energy, employing point charge representations of the active site, are quite useful. The dielectric behavior of the protein is much more complicated than is implicated in dielectric constant models; force fields that do not include an atomic level representation of electronic polarization are inadequate.

The a(R) = .Int. R/0 y ay dW r^{2} dr curves are presented for different contribution a to the energy of AOs. While all radial shells contribute about equally to the nonrelativistic kinetic and potential orbital energies, there is almost perfect cancellation of these energies in the inner shells, and the total energy of an orbital is almost solely determined by its outermost shell. In contrast to this, the first-order relativistic mass-velocity. Darwin and spin-orbit energies originate from the innermost shells only, while all radial shells contribute to the so-called indirect relativistic orbital energy correction. The indirect effect is important also for s AOs except for the central columns of the periodic system, where the indirect destabilization is compensated by indirect stabilization. This explains the 'gold maximum' of relativistic corrections. The results of this work offer a rationalization of the finding that the relative relativistic corrections approximately (Z/c)^{2}. are independent of electronic shielding or principal quantum number, while the non-relativistic orbital energies are approximately (Z_{eff}/n)^{2}. Conclusions on valence-only methods are also drawn.

Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

A quasirelativistic method, in which the valence density is optimized with respect to the first-order relativistic Hamiltonian, has been evaluated by calculations on systems containing heavy elements including third-row transition metals and actinides. The method adopts the statistical energy expression and employs in addition the frozen core approximation. The quasirelativistic method has been applied in calculations on AO energies for the valence shells of heavy elements. The quasirelativistic scheme gives results in better accord with the fully relativistic Dirac-Slater method than the first-order relativistic method based on perturbation theory. Calculations on the M-X bond energies in MX_{4} (M = Th, U; X = F, Cl, Br, I) as well as the M-R bond energies in Cl_{3}MR (M = Th, U; R = H, CH_{3}) revealed in addition that bond energies based on the quasirelativistic method (QR) were in better agreement with experimental data than bond energies based on the first-order perturbation theory (FO). The absolute mean deviations with respect to experimental values were 6.9 and 16.5 kcal/mol for QR and FO, resp., in the case of the MS_{4} systems. The quasirelativistic method, in which changes in the electron density induced by relativity are approximately taken into account in the energy expression, should be used for compounds containing actinides. Both QR and FO are appropriate for elements up to Z = 80, although QR represents a slight improvement for the elements in the third transition series. The calculated bond energies included contributions from a nonlocal correction to the statistical exchange potential as well as a potential representing correlation between electrons of different spins.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Preliminary molecular dynamics simulations of the nematic phase of 4-pentyl-4'-cyanobiphenyl are described. The simulations include all molecular degrees of freedom. The influence of the molecular dipole moment was investigated by comparing simulations with and without a charge distribution on the molecules. Inclusion of the charge distribution leads to a slight broadening of the orientational distribution function, in qualitative agreement with Raman measurements of the orientational order parameters.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

EPR and ENDOR studies on Si:^{105}Pd^{-} and Si:^{61}Ni^{-} are reported. The observations provide evidence for the transition metal to be a substitutional d^{9}-ion, bound to 2 Si neighbors. This model is proposed for Si:Ni^{-}, Ge:Ni^{-}, Si:Pd^{-} and Si:Pt^{-}. The identification is reported of Pt(II) as PtFe^{-} and the observation is described of its analog PdFe^{-}.

For a two-electron system the Kohn-Sham potential of density-functional theory is equal to the effective local potential V_{eff}(x1) occurring in the one-electron Shcroedinger equation that is satisfied by the square root of the exact many-electron density, r^{1/2}(x1). Making use of the theory or marginal and conditional probability amplitudes, is is shown that V_{eff}(x1) is the sum of three potentials, each of which has a clear physical interpretation and will be studied in detail. The correlation part of the Kohn-Sham potential in a two-electron system can then be obtained by subtraction of the Coulomb and exchange potential, and it is shown how we can express this correlation potential as the sum of three physical meaningful contributions. The connection between the Kohn-Sham potential in a many-electron system and V_{eff} is also discussed. Calculations of the various potentials from highly accurate CI wave functions are presented for the He atom and for the H_{2} at various distances of the two H nuclei.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Optical excitation energies ascribed to the Mn^{2+} center in ZnS:Mn are calculated from Gaussian-based SCF MOs obtained for the Th MnS_{4}^{6-} cluster at 3 Mn-S distances in a static external potential, followed by investigation of two levels of electron correlation effects. The 1st level includes only CI among the ligand field (LF) d5 states plus other intra-d-shell contributions by the empirical correlation-energy-correlation method (CEC), which is formally equivalent to the intermediate-crystal-field (CF) model. The 2nd level extends the CI to include a large manifold of cluster S to Mn relaxes charge-transfer (RCT) states. At the LF-CI + CEC level, the relative separations among, and the overall width of, the 6 lowest quartets agree with the 4 bands observed in the spectrum. Contrary to usual assignments, b^{4}T_{1}(^{4}P) is found below b^{4}T_{2}(^{4}D), and all quartets are obtained approximately 0.5 eV too high relative to ^{6}A_{1}. The delocalization effects in the MOs are smaller than obtained from empirical CF fits to the spectrum. At the 2nd level, CI mixing with the RCT states introduces important changes and interactions not properly encompassed by the orbital-based LF parametrization schemes. This CI depresses the quartet levels almost uniformly down by 0.2 eV relative to ^{6}A_{1}. The order of b^{4}T_{2} and b^{4}T_{2} and b^{4}T_{1} is reversed and now agrees with the usual assignments. These various results are discussed in comparison to less quantitative models in which extended interactions with the host are considered. A degree of similarity between the two formulations is established which gives further justification to the model as used.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Analysis of 4d core x-ray photoabsorption spectra (4d-XAS) in CeO_{2} was made with the impurity Anderson model by incorporating the solid-state effect of hybridization between 4f and valence-band states into the atomic calculation of multiplet structures. The hybridization effect plays an essential role in the multiplet structure observed in the prethreshold region of 4d-XAS. The effect of the finite width of the valence band and of the core-hole potential is discussed. The multiplet structures in a- and g-Ce were calculated.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The negatively charged state of substitutional Pt in Si is observable by ESR. The g-tensor of the ESR spectrum (labeled Si-Pt[I]) reveals orthorhombic-I symmetry of the center. The principal g-values deviate significantly from the pure spin value g = 2.0023, indicating substantial contributions from orbital momentum. The g-tensor data were analyzed on a model of one electron, with spin S = 1/2, in an orbital triplet state, L = 1. Spin-orbit coupling and crystal field interactions of cubic, tetragonal or orthorhombic symmetry were included in the model. The theoretical anaysis can account in a satisfactory manner for the experimentally observed values. The electronic structure of Pt^{-} is concluded to be the 5d^{9}6s6p configuration. This is consistent with predominant bonding of platinum with two silicon neighbors and dihedral distortion. The results are incompatible with alternative models, such as the vacancy model or a 5d-version of the Ludwig-Woodbury model. The orbital g-factor is reduced by about 30% by covalency.

A study is reported of the quantum chemical aspects of the proton transfer. A comparison of several basis sets is made in order to establish the minimal computational level at which the results for the proton transfer energy curve agree with those of more extensive basis set calculations. In order to obtain an accurate model for describing the effect of the environment on proton transfer, several interaction models are evaluated. The energies and charge density obtained with the Point Charge model, the Average Reaction Field and the Direct Reaction Field are compared. Also a comparison is made of energies obtained by the variational method and by first-order perturbation theory, resp. The Point Charge model and the Direct Reaction Field are used to decompose the calculated effect of the protein environment in terms of elements of primary, secondary and tertiary protein structure. The theoretical model is extended with a model for the solvent, surrounding the protein. The solvent is represented by a dielectric continuum and the space enclosed by the solvent accessible surface is used as the boundary of the molecular cavity.

The bonding and electronic structure of actinocenes M(COT)_{2} (M = Th, Pa, U, Np, Pu; COT = cyclooctatetraenyl) were studied using the relativistic HFS LCAO method. Nonrelativistically the 5fd AO is the most important AO for bonding, but in the beginning of the series (Th, Pa) the 6d also makes a very significant contribution to both d and p bonds. Relativistically the 6d contribution is even dominant in the beginning of the series. Experimental data, in particular photoelectron and electronic absorption spectra as well as magnetic susceptibility data (effective magnetic moments), are compared with the calculations and are consistent with this bonding picture. The crystal field models used to interpret the ground and excited states have incorrectly assumed a weak field approximation to be adequate. The strong interaction of 5fd with ligand levels causes the 5fd to be split off from the 5f manifold. The effects of spin-orbit coupling and electron-electron repulsion within a degenerate set of only fs, fp, ff orbitals provide an explanation of the observed excited states of U(COT)_{2}. The calculated magnetic moments of the lowest states are consistent with magnetic susceptibility data.

Conditions are discussed under which extended Koopmans (EK) eigenvalues become exact ionization potentials (IPs). Second-order perturbation theory is used to compare with known expressions involving relaxation contributions to the exact results. In two-electron systems all the EK eigenvalues are exact provided there are no vanishing natural occupancies in the exact reference state. In a finite basis model the EK eigenvalues do not match the exact IPs of the corresponding model Hamiltonian.

No abstract available

No abstract available

It is shown how the properties of the one-particle Green function lead naturally to the definition of the so-called natural energy orbitals. These orbitals allow the fully correlated total energy of a system to be written in Hartree-Fock-like fashion and might therefore provide a bridge between sophisticated correlated wave functions and approximate theories of chemical structure and reactivity based on a Hartree-Fock-like energy expression. Moreover these orbitals form the basis for a self-consistent scheme to calculate the one-particle Green's function. The relation between these natural energy orbitals and the extended Koopmans theorem is considered. The exactness of the lowest extended Koopmans ionization potential implies the linear independence of the corresponding Dyson orbital from all other Dyson orbitals.

The application of molecular ab initio methods to investigate the electronic structure of localized impurities in semiconductors requires the study of the convergence of the results with increasing cluster size. Results are compared for interstitial Ti in Si, obtained with clusters of increasing size: TiSi_{10}H_{16}, TiSi_{30}H_{40}, and TiSi_{66}H_{64}. These clusters contain one, two, or three shells of Si atoms, resp., centered around Ti at a Td interstitial site. The H atoms serve as saturators of the dangling bonds. The Si core electrons are replaced by an effective potential. The calculations are based on open shell RHF theory and limited CI extensions. The charge distribution in the central part of the three clusters is very similar. In the clusters the partially occupied orbitals are much more delocalized than the 3d orbitals in the free ions. The total impurity-induced electronic charge, however, is quite localized, due to the compensating response of the Si closed shell density. Ionization of the impurity also causes a compensating response of the Si closed shells: only about 10% of the density difference is in the impurity region and the major part is behind the outermost shell of Si atoms. Transition metal associated (3d-like) excitation energies are not very dependent on the cluster size, and the relative ordering of the lowest lying states remains unchanged. Impurity associated ionization energies decrease considerably due to the extra relaxation offered by the additional shells of Si atoms. The results indicate that a reliable description of interstitial transition metals in silicon can be provided by calculations on reasonably small clusters: Si_{30}H_{40} is sufficiently large.

By calculations on CuCl_{4}^{2-}, CuBr_{4}^{2-}, and NiO_{6}^{10-} clusters, 1st-order CI calculation improves the d-d and charge transfer (CT) spectra of ionic transition metal compounds. The 1st-order CI introduced delocalization (covalency) effects of the d

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

A review with 10 references on recent developments in the magnetic x-ray dichroism of rare-earth materials. The application of this technique to the study of magnetic materials is discussed. Also, other work on magnetooptical effects in the x-ray range is reviewed.

The single particle Green's function contains more detailed information than the total energy alone, to the extent that the local Slater-Löwdin correlation potential can be obtained from it. This potential can be used as a more detailed criterion to judge the quality of approximate Green's functions than the total energy by itself. The formalism leads moreover to a natural partitioning of the correlation energy into terms of depending on the correlation to the one density alone and a remaining "true" correlation contribution. The single particle Green's function is calculated using the second order approximation to the self-energy for a series of small model systems (He, Be, H_{2}, LiH, and H_{2}O). The correlation potential and the partitioning of the correlation energy are used to analyze this approximation and to assess its accuracy in these systems.

Physical and Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

Experimental dipole couplings for several isotopically substituted modifications of acetylene dissolved in the nematic liquid crystals 1132 (Merck ZLI 1132), EBBA [N-(4-ethoxybenzylidene)-2,6-dideutero- 4-n-butylaniline] and a 55 wt.% 1132/EBBA mixture are obtained from NMR measurements. The dipolar couplings are calculated using a model for the solvent-solute interaction. The interaction is taken to be of second rank tensorial form and the effects of the coupling between the vibrations and rotations are taken into account. The calculated values agree with the experimental results although some discrepancies exist. These discrepancies are discussed in terms of a possible contribution to the observed carbon-carbon coupling from the anisotropy in the indirect coupling. The results show that the interaction between the vibrations and rotations of the solute plays an important role in determining the observed couplings and that specific interactions with the liquid crystal need not be invoked to explain the results. Previous studies have shown that molecular hydrogen experiences an external electric field gradient due to its liquid crystal environment and that in 55 wt.% 1132/EBBA the value of this field gradient is zero at 301.4 K. It has also been shown that the interaction between this field gradient and the solute molecular quadrupole moment dominates the ordering of molecular hydrogen in liqid crystals, such as 1132 and EBBA, where the field gradient is large. A qualitative analysis indicates that this interaction is important for the ordering of acetylene and that other mechanisms also play a role.

The undistorted line shape in x-ray absorption may be obtained by analyzing the total photoelectric yield from solids as a function of the angle of incidence. This method is applied to the La3d absorption spectrum of LaF_{3}. An Auger half-width of G = 0.22 eV was found for the La 3d^{9} 4f^{1} levels, whereas the autoionization from the 3d^{9}3/2 4f^{1} level to the continuum gives an additional broadening of G = 0.3 eV.

Geometry optimization calculations were performed for UF_{4}, UCl_{4}, ThF_{4}, and ThCl_{4} using the relativistic Hartree-Fock-Slater method. These molecules have tetrahedral equilibrium geometries in their ground states. The U compounds have an open-shell ground state and could exhibit a Jahn-Teller distortion away from tetrahedral symmetry. Assuming a tetrahedral geometry for these molecules, vertical ionization energies were computed and this has led to a partial reassignment of the molecular gas-phase UPS.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Magnetic susceptibility and electron spectroscopic results are reported for a diluted alloy of U and Au, Au-U 0.85%. The data indicate localized U states with an effective magnetic moment of 3.1 m B per U atom.

A molecular model for electronic polarization of water is defined, consisting of interacting point dipole polarizabilities in an electric field generated by atomic point charges, which represent the gas-phase dipole moment of water molecules. The induced dipole equations are solved self consistently. The model is implemented in a Monte Carlo hydration simulation program, and its computational performance is discussed. Results of calculations of hydration and protonation energies of amines (including glycine) are presented and discussed, together with results for the water dimer and for liquid water. Solvent induction appears significant to describe quant. solvent screening of interactions between charges on the solute. In combination with solvent polarization a quantum chemical charge model is found superior to an empirical set of charges. Inadequacies of the Lennard-Jones type modeling of nonelectrostatic interactions between water molecules are demonstrated.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

A feasibility study of the application of the recently discovered strong magnetic x-ray dichroism of rare earth materials to the production of circularly polarized x-rays is reported. A device is described that can be inserted downstream from a high resolution double beryl crystal monochromator. Calculations show that 45% transmission can be obtained with filters that yield 99% circular polarization in the energy range 950-1500 eV. Advantages of the proposed device are the low costs, the ease of installation and the high product of transmission times polarization.

The fraction of the total line strength of 1 of the core-hole spin-orbit-split manifolds in x-ray absorption is proved to be related to the expectation value of the valence-band spin-orbit operator. The relation obtained, by angular momentum algebra, is valid if the total angular momentum of the core hole is a good quantum number, which is true for deep core levels. The branching ratio of deep-core-hole manifolds is probably the most direct probe to measure the valence-band spin-orbit interactions, especially in transition metal compounds. The branching ratio measures the angular part of the spin-orbit operator and is complementary to magnetic measurements which determine the Lande g factor.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Atomic calculations were made for the recently discovered magnetic x-ray dichroism (MXD) displayed by the 3d x-ray-absorption spectra of rare-earth compounds. The spectral shapes expected at T = 0 K for linear polarization parallel and normal to the local magnetic field are given, together with the temperature and field dependencies of the intensities of the spectral lines. Strong MXD effects can be expected for all rare-earths ions except those having a ground state with either J = 0 or L = 0.

High-resolution x-ray absorption spectra of high- and low-spin compounds were analyzed using a newly developed general crystal-field computer program. The calculations show clearly that the gross features of the multiplet structure at the L_{2,3} absorption edge can be used to determine the spin state of the 3d^{8} configuration in Ni and Cu compounds.

The origin of nonstatistical branching ratios in spin-orbit-split x-ray absorption spectra is explained. Atomic calculations for transition metals show a systematic change which is due to initial-state spin-orbit splitting and electrostatic interactions between core hole and valence electrons. The results of these atomic calculations are given in general rules, which are also applicable to solids. In the free atom, the branching ratio reaches a maximum for the Hund's-rule ground state and its value decreases gradually for S, L, and J levels of higher energy. The presence of a crystal field results in a lower branching ratio when it produces a low-spin ground state. The rules can be used to assess the spin state and the spin-orbit splitting from the experimental branching ratio in transition-metal and rare-earth compds. A specific example is given for the influence of second-order spin-orbit interactions in high-spin Ni compounds.

The Mott-Hubbard gap U and the charge-transfer gap D of solid NiO are estd. from ab initio calculations on the NiO_{4}^{10-} cluster. Covalency in the essentially localized dn states and localization for the spatially extended O(2p) hole states are introduced by means of a limited CI calculation. The localized states induced a large polarization effect in the bulk, accounted for in a semiempirical way. The values obtained for U and D are quite similar and in the range of 4.4-5.2 eV, in good agreement with the observed gap.

The branching ratio of core-valence transitions in x-ray absorption spectroscopy is linearly related to the expectation value of the spin-orbit operator in the valence states. This offers a direct method to determine the spin-orbit interaction in the local electronic structure of metal compounds and alloys. The method is complementary to susceptibility and paramagnetic resonance measurements because it measures a different operator and is element specific.

The localization of ligand-based valence holes in the tetrahedral complex ion [CrO_{4}]^{2-} in a crystaline environment is studied by SCF calculations on the hole states, with progressively less restrictions on the spatial symmetry of the MOs. The final wavefunctions are obtained by constructing, from the symmetry broken SCF solutions, wavefunctions that exhibit again the proper transformation properties under the operations of Td. The crystal environment of the [CrO_{4}]^{2-} anion is represented by a point charge model. In contrast with the situation for core hole states, the projection afterwards into Td symmetry is important. The final ionization energies, which are obtained from projected C_{3v} adapted SCF solutions, are reduced considerably (~3eV) with respect to the Td DSCF results, but the ordering of the states has not changed essentially. The calculated ionization energies compare favorably with results of XPS experiments on Na_{2}CrO_{4}. The evaluation of the energies of projected symmetry broken SCF solutions requires the calculation of Hamiltonian matrix elements between determinantal wavefunctions built from mutually non-orthogonal orbital sets. An efficient method for the calculation of such matrix elements is presented.

Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

A review with 24 references on the spectra of H_{2}, HD, and D_{2} solutes in nematic solvents.

The ^{2}P_{u} state of Br_{2}^{+} shows an unusually large difference between the vibrational frequencies of its two spin-orbit components. This can be understood from the combined effects of correlation and spin-orbit interaction, which is largely due to the partial localization of the hole in the ion. The same mechanism can be held responsible for vibrational and rotational anomalies in the corresponding state of Cl_{2}^{+} and F_{2}^{+}.

The use of a Gaussian charge distribution to represent the nucleus is advantageous in relativistic quantum chemical basis set expansion calculations. It removes the singularity at the origin of the Dirac wavefunction, leading to a more rapid convergence of the ground-state energy expectation value as a function of basis set size and to a large reduction in the exponents of the optimized basis sets.

No abstract available

no abstract available

A simple model was developed to represent long-range electrostatic interactions in computer simulations of solvated molecules and ions. The model combines a discrete molecular description of the first two or three solvation layers with a continuum description of the bulk solvent. The solute is described quantum mechanically. For a series of amines good agreement with experimental protonation energies in water is obtained, when the position of the continuum boundary takes properly into account the volume of the enclosed particles. A comparison is made with periodic boundary simulations and with existing continuum models. The accuracy depends not only on the dielecectric model itself, but at least as much on details of the electrostatic potential and on inductive interactions.

Matrix representations of the Dirac equation give stable positive energy eigenstates if the large and small component bases are kinetically balanced. Results are presented for relativistically optimized basis sets using a Gaussian charge model for the nucleus. Some recent results for many-electron systems are compared with previously obtained results of other authors.

no abstract available

no abstract available

Ab-initio, all-electron, SCF plus CI calculations were done on some simple models of dichloro- and difluoro-bridged Cu(II) dimers. The splitting between the lowest singlet and triplet energy levels is strongly dependent on the angle f (the Cu-L-Cu angle) between the copper ions and the bridging ligands L. The singlet-triplet splitting ES - ET shows a maximum for f between 90 and 100 degree. For angles in this region, the triplet state is lowest; whereas for larger and smaller angles, the singlet state is lowest. The strong dependence of the splitting originates predominantly from 1st-order, copper-to-copper, charge-transfer contributions to the singlet wave function.

Gaussian basis sets for atomic one-electron systems have been optimized by straight *minimization* of the electronic ground-state eigenvalue of the finite basis set representation of the Dirac operator, using the "kinetic energy balance" procedure in conjunction with appropriate additional variational freedom. The results, which are apparently upper bounds, are presented and compared with previous data.

No abstract available

No abstract available

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

A "double zeta" basis set ab initio method was used for the investigation of the systems (trimethylamine-dimethylphosphate mono anion)H^{+} (i), aniniline-dimethyl phosphate monoanion (II) and formanilide-dimethylphosphate monoanion (III), which represent models for associative sites of both local anaesthetics and the phospholipd part of the nerve membrane. According to the authors' calculations, complex I was found to be the most stable with a N^{+}-H.....O^{-} hydrogen bond. Further, the PCILO method was used for the investigation of the interactions of the polar groups of 1-[2-(2-methoxyphenylcarbamoyloxy)ethyl]piperidine (B) and its cation (BH) with N-methylacetamide, which represents a model of the association sites of the lipoprotein part of membranes. The strongest hydorgen bond with the carbonyl group of N-methylacetamide forms a N^{+}-H group of cationic form.

A review with 35 references on the relevance of ab initio quantum mechanical calculations to enzyme chemistry. A method to extend such calculations to the domain of solvent effects and the theoretical investigation of chemical reactions in the active site of a protein is discussed.

Results are presented of a Hartree-Fock cluster study of interstitial Ti, V, Cr, and Mn impurities in Si. A Si_{10} cluster modeled the nearest Si atoms around a tetrahedral interstitial site in crystaline Si. The dangling bonds of the Si atoms were saturated by hydrogens. The effect of the Si core electrons was represented by an effective potential. Characteristic for the electronic structure, of the low-lying states of the neutral, singly positive, and doubly positive ions in silicon, was the presence of fairly delocalized but still predominantly transition-metal (3d)-like orbitals of t_{2} and e symmetry. For all ions, the energy of the weighted average of the terms belonging to a configuration was lowest for the configuration with maximum occupation of the t_{2} orbitals. Ground states with maximum spin multiplicity were found for all ions, except Ti^{0}.

In an extension of previous work (A. and N., 1985), a properly balanced Gaussian basis was used to solve the Hartree-Fock-Dirac- Roothaan SCF equations for many-electron systems. All calculations were done with the relativistic molecular program package MOLFDIR, exploiting molecular double group symmetry to reduce integral storage requirements and to obtain a Fock-matrix for each symmetry representation. Closed-shell SCF results are given for the energies of various atomic and molecular systems.

Results are presented of a Hartree-Fock plus limited-CI-cluster study of the 3d transition metal impurities (from Sc to Co) at the Td interstitial site in silicon. The cluster was Si_{10}H_{16} and the Si core electrons were represented by an effective potential. Low-spin ground states were found only for Sc^{-}, Ti^{-}, Ti^{0} and V^{-} Interelectronic repulsion dominates over the pure crystal-field effect. A modified crystal-field anal. of the data required different orbital-deformation parameters for configuration average energies and term splittings. In bare substrate clusters, the electronic energy levels behave quite regular as a function of cluster size. Surface effects in the charge distribution have been detected.

It is shown that the restrictive conditions of Wood et al. [1] are not necessary to reach the conclusion that the Dirac hamiltonian, projected onto the space of the large component, exhibits variational properties. The eigenvalue spectrum of matrix *approximations* to the partitioned hamiltonian (obtained by matrix partitioning) coverges to the exact spectrum in the limit of infinite order (assuming completeness) but not necessarily from above as for true matrix representations (obtained from operator partitioning). Optimization of non-linear parameters is shown not to cause variational instabilities.

No abstract available

Recently, it was suggested that parallel b-sheets have a significant dipole moment, in contrast to antiparallel sheets. Ab initio MO calculations on parallel and antiparallel b-strands of (Gly)_{4} show that they have very similar charge distributions. Interaction energies between 2 and 3 strands of (Gly)_{4} obtained by the direct reaction field Hamiltonian, show that a particular choice of point charges is probably not crucial for calculating interactions within b-sheets, but that it might be significant for calculating interactions between these sheets and other parts of a protein, in particular, a-helixes. The point-charge representation of the MO-SCF results will probably reduce the hazard of introducing artifacts in electrostatic calculations of protein conformational energies, provided the short-range interactions are treated in a more realistic way, i.e., such that intra- and interchain induction effects are included.

The solutions of the matrix representation of the Dirac equation obtained by expansion in Gaussian basis sets are examined. The basis sets consist of nonrelativistically energy-optimized Cartesian Gaussians, properly balanced by a basis set constraint, or a generalized modified [s.p] representation. The quality of the solutions is illustrated by calculating the expectation values of various radial moments in addition to the energy eigenvalues. An expression is given for Gaussian contraction coefficients., consistent with the basis set constraint.

No abstract available

A review with 19 references is given of the general theory for the orientation of small molecules dissolved in liquid crystal solvents. Recent experimental results for H_{2} and CH_{4} and their deuterated analogs in a number of thermotropic phases are discussed in terms of this theory. Physical mechanisms responsible for solute-solvent interactions are discussed.

Physical and Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

Dipolar and quadrupolar couplings are reported for CH_{4} and its deuterated analogue dissolved in the liquid crystals Merck ZLI 1132, N-(p-ethoxybenzylidene)-p'-n-butylaniline, and a mixture of these liquid crystals. Previous studies indicate that D_{2} dissolved in the same mixture has almost zero orientational order. This was interpreted as a reduction in the average electric field gradient on mixing. CH_{4} in the same mixture and in its constituents experiences the same average field gradients as D_{2}. Provided one does not neglect the variation of the relevant molecular tensors with bond stretching, the dipolar couplings of CH_{4} dissolved in liquid crystals can be described with a bond-additivity model.

SCF calculations were done on the ground states, ionized states, and some excited states of the clusters [CuCl_{4}]^{3-}, [CuBr_{4}]^{3-}, [Cu_{4}Cl]^{3-}, and [Cu_{2}Cl]^{+}. The Madelung field of the rest of the crystal was incorporated in the calculations. High and low point-group symmetries were considered, showing the importance of relaxing symmetry constraints. The calculated ionization energies agreed satisfactorily with existing experimental data; however, the calculated crystal-field splitting of the 3d levels was much smaller than the observed one. The calculated 3d-4s-like excitation energies were very sensitive to both the external field and the basis sets used. The results indicated that, at the level of the single-configuration SCF approximation, open-shell orbitals describing 3d holes are strongly localized on copper. Their covalency is small, so that a seemingly necessary contribution to an understanding of the spectral properties is missing at this level of approximation, which includes one-electron band theory.

The results of an ab initio all-electron Hartree-Fock study of the interaction of O with Al_{10} and Al_{13} clusters are reported. These clusters model the close-packed Al(111) surface. The Al_{10} cluster has 7 atoms in the first layer and 3 in the second; the Al_{13} cluster has only 3 atoms in the first layer. The chemisorption of atomic O at the fcc sites of Al(111) was studied by calculations on Al_{10}O_{3}, Al_{10}O, and Al_{3}O. The Al_{10}O_{3} cluster was chosen to provide the opportunity of incorporating part of the possible influence of O-O interactions on the chemisorption properties. A stable phase of O was found to be chemisorbed just outside the surface, with a vertical distance between O and the first Al plane of approximately 0.6 A. The calculated vibrational energy of the O motion in a direction normal to the surface in the harmonic approxn. is approximately 50 meV. The results are consistent with the coexistence of O in overlayer and underlayer configurations. No significant influence of O-O interaction on the calculated properties was found. Finally, the calculation of the chemical shift of surface Al(2p) ionization is discussed.

Ab initio calculations on the electronic structure of tetrahedral clusters [CuCl_{4}]^{3-}, [CuBr_{4}]^{3-} and [Cu_{4}Cl]^{3+} and the [CuClCu]^{+} cluster of C_{2v} structure embedded in the Madelung field of the rest of the crystal show that the simplest orbital model, the single configurational spin-RHF approximation, leads to a localized rather than a delocalized 1st order description of states involving a 3d-hole in the 3d^{10} compounds. CuCl and CuBr. The calculated photoionization spectrum is in reasonable agreement with the experimental data. The covalency effects in the 3d-hole states that are inferred from photoionization and optical data are not obtained on the Hartree-Fock level of approximation, but result from CI between 3d-hole states and anion np-hole states.

Improved experimental dynamic deformation density maps of the title compound were compared with theoretical static density maps computed for a molecular cluster with an extended basis set. No conclusive evidence is obtained on a possible polarization of the O lone-pair electrons towards the H bonds.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The quantum-chemical PCILO method was used to perform a conformational analysis of the 1-[2-(2-methoxyphenylcarbamoyloxy)ethyl]piperidine (I) [76875-80-4] and its cation (I-H^{+}) which belong to a group of phenylcarbamates with considerable local anesthetic activity. For I the most stable conformation has a gauche arrangement of the O-C-C-N fragment. For I-H^{+} the most stable conformer is stabilized by an intramolecular hydrogen bond of the N^{+}- H.....O=C type. Using the PCILO and double zeta ab initio methods the protonation energies were calculated The gross atomic charges resulting from the double zeta ab initio calculations for I and I-H^{+} were compared with the results of the PCILO charge distribution analysis.

Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

Vertical ionization energies from the outermost 2 p_{u} and 2 s_{u} MO's in the diatomic halogen and interhalogen molecules were calculated by using a relativistic Hartree-Fock-Slater (HFS) method. The results obtained are in good agreement with the corresponding experimental ionization energies obtained from photoelectron spectroscopy. The spin-orbit splittings of the ^{3}P_{u} cationic states are rationalized in terms of the degree of localization of the partially filled p orbital in the cation on the heavier atom.

The idea of P. D. Dacre (1970) and M. Elder (1973), concerning the reduction of the two-electron molecular integral file by using symmetry, can be used in relativistic Hartree-Fock-Dirac calculations on molecules containing rare earth atoms without reformulation. The integral calculation can remain nearly unchanged, hence the advantages of the algorithm are fully conserved.

Results of calculations are reported for UO_{2}^{2+} using the LCAO MO Hartree-Fock-Slater method including relativistic effects. The HOMO is calculated to be s_{u} consisting predominantly of U 5f character. This s_{u} orbital is the HOMO partly because of "pushing-from-below" by the U 6p orbital, but also as a result of the change in potential of the U 5f electrons with the U core electrons brought about by relativistic contraction of the core electrons. This effect also determines the character of the 1st virtual levels (d_{u} and f_{u}, resp.) in equatorial ligand fields.

Physical and Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

NMR spectra of D dissolved in mixtures of the nematic liquid crystals N-(p-ethoxybenzylidene)-p'-n-butylaniline and Merck ZLI 1132 are discussed in terms of a theory based on the interaction of the molecular quadrupole moment with the average electric field gradient. The average field gradient depends upon the liquid crystal composition and hence can be adjusted to various values (including zero) between those of the pure liquid crystals. For D dissolved in these phases the molecular quadrupole-average field gradient mechanism can account for most but not all of the orientational ordering.

A review with 112 references is given on the discrete variational Xa method, known also as the LCAO Hartree-Fock-Slater method, accounting for relativistic effects and its use for calculating ionization potentials.

No abstract available

No abstract available

The evaluation and processing are discussed of integrals over AO's using the LCAO model.

The feasibility of the inclusion of reaction field effects in accurate ab initio SCF-MO calculations was studied in the case of proton transfer in the active site of actinidin. The effects of the polarizability of the environment were included, by using the direct reaction field model, which treats the environment as a set of interacting polarizable atoms. Up to 1000 of these atoms could be treated but .approximately 300 were sufficient. The full geometry of the active site and the environment was taken into account. The stabilization of the ion pair was calculated to be 3.5 kcal, but this value may be 10 kcal depending on the geometry used. The effect of the static field from the long a-helix present in the enzyme was also studied. Dispersion effects were shown to be unimportant. The orientational polarizability of side chains and water molecules was not included.

Valence ionization energies of the transient species GeI_{2}, obtained with He I photoelectron spectroscopy, are presented. The interpretation of the results is aided by Hartree-Fock-Slater calculations in which relativistic effects were taken into account through perturbation theory. A comparison with data of the other Ge dihalides is made.

Physical and Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

The difference between liquid-crystal and gas-phase values for the nuclear quadrupole coupling constant in D_{2} and HD was used to obtain the mean electric field gradient in various liquid crystals. Order parameters for small molecules dissolved in liquid crystals were calculated assuming that the orientational order arises from the interaction of the molecular quadrupole moment with the average field gradient. The results obtained are in good agreement with experimental values for H and several other solutes.

Three different algorithms for the calculation of many center electron-repulsion integrals are discussed, all of which are considered to be economic in terms of the number of arithmetic operations. The common features of the algorithms are as follows: Cartesian Gaussian functions are used, integrals are calculated by blocks (a block being defined as the set of integrals obtainable from four given exponents on four given centers), and functions may be adopted to R(3). Adaptation to molecular point group symmetry is not considered. Tables are given showing the minimum number of operations for a selection of block types allowing one to identify the theoretically most economic, and the corresponding salient features. Comments concerning the computer implementations are also given both on scalar and vector processors. In particular, the Cyber 205 is considered, on which the more efficient algorithm was implemented.

A general molecular theory for the description of the orientation of small moleculaes in anisotropic environments is presented. Assuming a 2nd-order tensorial interaction between some solute property b_{ij} and the anisotropy in a liquid-crystal field F_{ij}, rigid and non-rigid contributions to the dipolar and quadrupolar couplings observable by NMR were evaluated. The behavior of the solutes H_{2} and CH_{4} and their deuterated analogs in nematic phases can be described by special cases of this general theory; the observed and calculated anisotropic couplings agreed. The coupling between the solute molecular quadrupole moment and the anisotropy in the liquid-crystal electric field gradient plays a significant role in the orienting process in the case of H_{2}.

D quadrupolar couplings obsd. when the deuterated methanes are dissolved in liq. crystals can be understood on the basis of the same vibration-rotation coupling mechanism which can explain the measured dipolar couplings. The elec. field gradient at the site of the D nucleus when the deuterated methane possesses its equil. geometry; and the deriv. of this field gradient with respect to the F_{2} stretch symmetry mode, taken at the equil. geometry are obtained. To obtain consistency, the presence of an external elec. field gradient in the liq.-crystal solvent is required and its magnitude is estd.

Elec. field gradients in the molecules H_{2} and CH_{4} are calculated by the Hartree-Fock and Hartree-Fock-Slater methods. In CH_{4}, the derivatives of the electric field gradient with respect to the vibrational symmetry modes are determined. One C-H stretch symmetry mode derivative is completely dominant. This derivative has recently been used to explain the quadrupole splittings found in the NMR spectra of the deuterated methanes dissolved in liquid crystals.

The chemisorption of O on Li, Al, Ni and Cu surfaces was investigated using the ab initio Hartree-Fock cluster model. These substrates have the possibility for different bonding in that Li is a simple s metal, Al an s,p and Ni(Cu) an s,p,d metal. Binding energy curves were calculated as a function of the O-metal distance. By using these curves, O-metal normal vibrational frequencies, and the equilibrium bond distance were calculated. The calculated vibrational energies were compared with electron energy loss spectroscopic (EELS) data for Al and with satisfactory agreement. O adsorbed on Ni(100) for which coverage dependent loss peaks have been reported but no generally acceptable interpretation exists to data are discussed.

A review with 21 references. Emphasis is placed on enzyme model calculations.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

A double zeta basis set ab initio SCF MO method has been used to study intermolecular hydrogen bonding in the systems [trimethylamine-dimethylphosphate monoanion]-H^{+} (I) aniline-dimethylphosphate monoanion and formanilide- dimethylphosphate monoanion which represent the models for associative sites of both local anesthetics and the phospholipid part of the nerve membrane. Complex I was the most stable with an N^{+}-H...O-hydrogen bond and an interaction energy of 580.17 kJ mol^{-1}. The proton transfer in I was also investigated. The proton potential function calculated at distance RN...O = 0.269 nm showed a double-minimum.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

A minimal basis set ab initio SCF MO method has been used to find the stable conformations of the polar -COO-, -NH-CO- and, -NH-COO- groups in the ester in 4-aminobenzoate Me ester (I) [619-45-4], 2,6-dimethylacetanilide (II) [2198-53-0], and 2- methoxyphenylcarbamate Me ester (III) [619-45-4] which are models for the local anesthetics procaine [59-46-1], lidocaine [137-58-6], and heptacaine [55792-21-7] resp. For both I and III the most stable conformations were found to be the planar forms in which -COO- and -NH-COO- groups lie in the plane of the aromatic ring. For II the most stable conformer is nonplanar with the angle of rotation of the -NH-CO- group out of the benzene ring plane equal to 60 degree. Furthermore, the electrostatic molecular potential contour maps were evaluated for models I-III. The calculations show that the aromatic parts of these drugs possess large negative potential regions which are essentially a superposition of substituent nitrogen and oxygen atoms, as well as resulting from the p-electrons of the aromatic ring. Therefore, the parts of the local anesthetics investigated may act as electron donor sites in drug-receptor interaction.

An efficient method is proposed for obtaining atomic charges from molecular wave functions, preserving both total charge and dipole moment. The method is independent of the type of wave function (SCF, CI) and does not refer explicitly to the basis set used, nor to integral approximations (e.g. CNDO) applied. The method takes very little time and is better than Mulliken's analysis as a generator of electric potentials.

Some properties of a-helices of polyglycine and polyalanine, up to the decapeptide, were investigated by ab inito MO calculations. These helices were unstable relative to the corresponding fully extended chain conformation. The electric field of helices of 8-10 residues is about 20% stronger than that of models built from noninteracting monomers. This is a result of cooperativity, which is essentially governed by the intramolecular H bonds. The cooperativity is manifest in all properties of the helices: relative stability, dipole moment, proton affinity, and electric potential. The electric potential of helices of 3 and 4 residues is such that their instability can be compensated for by a single charged group acting as an initiator. The computed proton affinity of the (Ala)_{8} a-helix is about 45 kcal/mol larger than that of formamide, which confirms that long helices may be protonated at the carboxyl end in solution.

Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

The He I photoelectron spectrum of SnO(X^{1}S^{+}) was recorded. Two bands were observed corresponding to ionization from the 6p and 13s valence MO with vertical ionization energies of 9.98 and 10.12 eV, resp. Vibrational structure associated with the 1st band has been analyzed to give w_{e} = 700 40 cm^{-1}, R_{e} = 2.00 state. An assessment was made of the ability of Hartree-Fock-Slater calculations, multiple-scattering SCF-Xa calculations and ab initio DSCF calculations to predict ionization energies for the Group IV diatomic monoxide molecules.

Valence ionization energies of CI_{4} obtained with He(I) photoelectron spectroscopy are presented. The interpretation is based on the results of Hartree-Fock-Slater (HFS) calculations using Slater-type function basis sets of triple-zeta quality. Effects of relativity are taken into account through a perturbation treatment to first order in the square of the fine-structure constant. The agreement between experment and theory illustrates that the HFS scheme including a relativistic perturbation approach is a reliable method for the calculation of ionization energies of molecules containing heavy atoms.

The He(I) photoelectron spectrum of CI_{2}=CI_{2} is reported. The results are analyzed on the basis of Hartree-Fock-Slater calculations. Effects of relativity are taken into account through a perturbation scheme to first order in the square of the fine-structure constant. Experimental results and theoretical calculations agree.

The induction and dispersion terms obtained from quantum-mechanical calculations with a direct-reaction-field Hamiltonian are compared to second-order perturbation-theory expressions. The dispersion term gives an upper bound which is a generalization of the upper bound given by M. H. Alexander (1970). The model was used to calculate the interactions in the water dimer. The long-range Coulomb, induction, and dispersion interactions were satisfactorily reproduced.

Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

Photoelectron spectra are reported for BiCl_{3}, BiBr_{3} and BiI_{3}. Assignments were obtained by using the relativistically parameterized extended Hueckel (REX) and perturbative Hartree-Fock-Slater methods. The halogen p-bands (4a_{1}, 1a_{2}, 4e, and 3e) resemble those in the lighter MX_{3} molecules (M = P-Sb). The Bi-halogen s bond MOs (3a_{1} and 2e) suffer a relativistic rehybridization into e_{1/2} > e_{3/2} > e_{1/2}.

No abstract available

The topics reviewed with 24 references include: relativistic Hartree-Fock-Slater [RHFS] theory and comparison with the Dirac-Fock-Slater theory; and RHFS calculations of (a) orbital energies of heavy atoms (Au, Hg, Rn), (b) bond lengths, bond energies, and vibrational frequencies of heavy molecules (CuH, AgH, AuH, Cu_{2} , Ag_{2}, Au_{2}), (c) ionization potentials of heavy atoms and molecules (I_{2}, Hg, Xe, Rn, TeBr_{2}, HgI_{2}), and (d) electron densities in HgCl_{2}.

The interaction potential that describes the orientation of CH_{4} and its isotopic derivatives in a liquid-crystal environment has a simple 2nd-order tensorial form. The dipolar couplings previously observed in the methanes arise from a vibration-rotation coupling mechanism and a rigid-molecule effect present in the nonsymmetrically substituted methanes. These effects are calculated from the CH_{4} force field. A large part of the interaction can be identified with the coupling between the anisotropic liquid-crystal electric field and the (vibrationally induced) solute anisotropic polarizability. Relative sign information is obtained on the derivatives of the CH_{4} polarizability with respect to the symmetry modes, quantities which are crucial in the determination of absolute Raman intensities.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The dipole and quadrupole moments, diamagnetic susceptibility and 2nd moment anisotropies of the title molecules were determined by ab initio calculations. The results agree with experimental except for the dipole moment of C_{4}H_{4}S, which is very sensitive to the basis set used. Unmeasured 1-electron properties are also given.

Physical and Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

^{1}H and ^{2}H NMR measurements are reported for H_{2}, HD, and D_{2} dissolved in various nematic phases. Signs of order parameters are liquid-crystal dependent; the molecules with larger average bond lengths show smaller order parameters; the order parameters for ortho- and para-D are equal within experimental error; the ratios of quadrupolar to dipolar couplings in HD and D_{2} are about 7% lower than in the gas phase. These results are rationalized in terms of environmental effects of the liquid-crystal field on the dissolved molecules. This field decreases the quadrupolar coupling in HD and D_{2} and causes extensive mixing of different rotational states of the H in the liquid phase. The observed isotope dependence of the average orientation is a quantum-mechanical effect which can be explained by the effect of a simple one-parameter mean liquid-crystal field on the rotational energies and rotational wave functions of the H.

Some problems concerning the reaction mechanisms of papain and thiolsubtilisin (subtilisin in which the active-site serine O is replaced by S) are discussed. Results of ab initio SCF calculations, which contributed to the solution of those problems, are briefly reviewed. The environment of an enzyme's active site plays an important part in the activity. To include environmental effects in quantum mechanical calculations, the direct reaction field method, in which environment is modelled by a collection of point-charges and -polarizabilities, was used to give a reasonably accurate estimate of interaction energies up to 2nd order, in which dispersion is overestimated approximately 2-fold. Actual calculations on the water dimer and a papain model system show that it is possible to simulate, with this method, ab initio calculations on large systems.

The electric field of an a-helix is due to the alignment of individual peptide dipole moments parallel to the helix axis. The resulting field for a 20-residue helix is of the order of 109 V/m, i.e. about equivalent to that of a half unit charge. Some biological effects of the helix field are discussed, including regulation of enzyme mechanism, cooperativity in growing protein chains, driving cross-membrane ion pumps. Ab initio MO calculations on polyglycine and polyalanine showed that the actual field strength may be significantly larger than estimated from point charge models.

Basis-set expansions are presented for the Hartree-Fock Slater (HFS) orbitals of the neutral elements Fr-Lr (Z = 87-103). The Slater-type functions used in these expansions are found by an efficient fitting procedure to the Herman-Skillman numerical HFS orbitals. The expansions are of single-zeta, double-zeta, and triple-zeta-valence (extended) quality. Comparisons of orbital energies with the numerical values are given for all elements. Similar basis sets for all the remaining elements are available on request.

Based on computed proton affinities for several model systems, the energetics of proton transfer and the acidity of the catalytic triads Cys-His-Asn (papain), Cys-His-Asp (thiol-subtilisin), and Ser-His-Asp (subtilisin) are discussed. In papain the ion-pair Cys--HisH^{+} exists owing to the intramol. elec. field, and a similar situation is found in thiol-subtilisin, but not in subtilisin. Assuming similar reaction mechanisms for papain and thiol-subtilisin (proton transfer from HisH^{+} to the NH group of the scissile peptide bond), the inactivity of thiol-substilisin towards proteins is explained by the much greater basicity of His in the complex His-Asp- than in His--Asn. For this explanation to be consistent, it is tentatively concluded that the catalytic mechanism of the serine proteases is different from that of the cysteine proteases, and involves direct transfer of the serine proton to the leaving group in the acylation step.

The localization of holes in systems containing spatially equivalent sites is discussed in terms of a simple one-particle model in which quantum mechanical delocalization effects compete with essentially classical polarization or dielectric relaxation effects. The predictions of the model for a tetrahedral system like CrO_{4}^{2-} compare favorably with the results of symmetry unrestricted SCF calculations on O1s hole states. The connection with a CI treatment using symmetry-restricted MO's is discussed. The calculated ionization energies are finally compared with XPS measurements on Na_{2}CrO_{4}. To this end the crystal surrounding of the CrO_{4}^{2-} anion has been represented by a point charge model and the ensuing Madelung field was included in the SCF calculations. In contrast to the Td restricted result of 551.4 eV, the completely localized C_{3v} result of 532.6 eV is in satisfactory agreement with the experimental data which are found around 530.0 eV.

The point dipole interaction model for molecular polarizability recently proposed by J. Applequist, et al., (1972) is modified by replacing the point dipole interaction by an interaction between smeared out dipoles. Rules are developed to indicate plausible forms for this modified interaction. The polarizabilities of a wide range of chemically different molecules can be calculated, using for each atom one polarizability independent of its chemical environment. The errors are comparable to experimental uncertainty. Special care is taken to produce a model that tends to avoid infinite polarizabilities without use of cutoffs at short distances.

Minimal basis set ab initio SCF MO expectation values of the electrostatic potential of an alanine octomer a-helix are reported. The energy profile for a proton moving through the interior of the helix is given. The internal elec. field of a helix may be essential in biol. ion pumps.

In some metal hydride molecules, the 5d AOs diminish the usual relativistic bond-length contraction. This is a first-order effect, unrelated to the relativistic expansion of 5d AOs, as shown in calculations by both the perturbative Dirac-Fock one-center expansion and Hartree-Fock-Slater methods (T. Ziegler, et al., 1980-1; S. and P., 1980). The latter method was used to investigate the chemical similarity of Zr and Hf.

No abstract available

Ab initio MO calculation may be used as a supplement to or in place of experimentation or to obtain information not accessible experimental, e.g., on structure-function relations in biological systems such as the active sites of enzymes. The availability of computers as vector processors will make CI problems amenable to ab initio analysis.

No abstract available

No abstract available

No abstract available

A semiquantitative analysis of the large differences in rate for the intramolecular carboxyl-catalyzed hydrolysis of the sulfonamides I-V is described. Ab initio MO calculations, using a double-zeta basis set of contracted Gaussian orbitals, provide further support for a stepwise nucleophilic substitution at sulfonyl sulfur involving a pentavalent sulfur intermediate. Calculations. on the different conformations for this intermediate show a clear preference for a trigonal-bipyramidal structure with apical bonds to the incoming nucleophile and the amine leaving group. Perturbation of the ideal bond angles for these groups is associated with substantial increases of the energy, implying well-defined geometrical constraints in the formation of the transition state for hydrolysis. The extremely slow hydrolysis of IV is easily rationalized in terms of this theory. The preference for the formation of four- and five-membered rings rather than six-membered rings in this intramolecular process is also accommodated by the theoretical results.

A recently proposed perturbational approach to relativistic calculations on molecules was applied to a number of compounds containing heavy elements. Comparison of calculated spectroscopic constants with experiment and results from other theoretical models shows that the method is a viable alternative to more involved treatments. Relativistic corrections are essential for compounds containing heavy elements and result in a contraction of bond distances and a substantial change in bond energies. The perturbational approach lends itself readily to a straightforward interpretation of both effects. It appears that the bond contraction is not due to the relativistic contraction of valence atomic orbitals. It is a direct relativistic effect, the repulsion due to the rise in kinetic energy when bonds are shortened being diminished by the mass-velocity correction.

Laser-induced energy transfer was observed and studied in the system pentacene-doped naphthalene. The transfer spectrum shows a remarkable correspondence with the host density of states function. The rate for laser-induced energy transfer is given. Most likely, intermolecular exchange is the mechanism that determines the transfer.

A review with 31 references on ab initio MO calculations describing the effect of static fields on the active sites of subtilisin, papain, and thiol-subtilisin. Quantum chemistry can provide additional and independent evidence about many aspects of molecular catalysis.

van der Waals Laboratorium, Universiteit van Amsterdam, 1018 XE Amsterdam, The Netherlands.

Ab initio SCF MO calculations were carried out for the title molecules and the expectation values derived for electronic potential, dipole, quadrupole, octopole moments, diamagnetic-susceptibility 2nd moments, 2nd-moment anisotropies of the charge distribution, electric field, electic-field gradient, diamagnetic shielding, and nuclear quadrupole coupling constant. Extended basis sets including d-type polarization functions were used. Excellent agreement with experiment is obtained.

Constraints, i.e. bond length or angle constraints, are incorporated in the algorithms used in Brownian dynamics simulations of molecular liquids or solutions. The validity of the model, in which the stochastic and frictional force fields possess neither time nor space correlations between different atoms, is examined by comparing results for liquid butane and decane with those of molecular dynamics simulations. The model gives a good approximation to the molecular dynamics in the liquid state. For butane, solvent packing effects are important in the condensed phase. For decane, the equilibrium conformation and dynamics are determined mainly by intramolecular interactions. A correlation occurs between consecutive conformational transitions of both 1 dihedral angle and 2 dihedral angles due to conservation of angular momentum. A Kramers-modified transition state theory gives reliable transition rates.

The direct reaction field model (T. et al. (1980), was used to calculate the interaction of 2 H atoms and of 2 water molecules. The terms included in the Hartree-Fock energy for the dispersion interaction were analyzed.

A population analysis method is given for molecular wave functions. It preserves charge as well as dipole moment. The method does not explicitly refer to the orbital basis set or the particular form of the wave function.

A review with many references.

Open shell restricted SCF calculations on the sN, sO, pN, and pO states of HCONMe.bul. and MeCONMe.bul. and on the sN and pN states of MeSO_{2}NMe.bul. were carried out using the double zeta basis sets of Roos and Siegbahn (1970). The fact that the carboxamidyls possess higher A_{N} values (.approximately 15 G) than their sulfonyl analogs (A_{N} approximately 13 G) can be reasonably accommodated in terms of the quantum mechanical results. Whereas the ground state of the sulfonamidyls is adequately described by a pN configuration, the ground state of the carboxamidyls is a composite of p and s configurations. Therefore the MO containing the odd electron in the carboxamidyls will have higher S-character, mainly as a result of a contribution of the bent sN state. This may well lead to a relative increase in the nitrogen hyperfine splitting constant for the carboxamidyls despite the significant unpaired electron delocalization over the carbonyl function.

Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

The electronic structure of the complexes [Fe_{4}S_{4}(SH)_{4}]^{n} (I, where n = 0, 2-, 3-), which model the 4-Fe active site in high-potential Fe protein and ferredoxin, was calculated with the Hartree-Fock-Slater- LCAO method (in its frozen core and core pseudopotential versions). Results were in agreement with the measured electronic absorption spectrum and magnetic behavior. The electric field gradient on the Fe nuclei was larger than expected from the observed Moessbauer quadrupole splitting, but the (small) change in this quantity in going from the dianion to the trianion was well described. The Fe-S bonding was mainly covalent and direct Fe-Fe bonding was weak; these conclusions also followed from extended Hueckel calculations which were made in parallel. The self-consistent HFS-LCAO calculations showed that in redox reactions the Fe atoms act as charge redistributors. The core pseudopotential version of the method yielded results which generally agreed with those of the frozen core calculations.

Valence ionization energies of the transient species TeCl_{2} and TeBr_{2}, obtained with He I photoelectron (PE) spectroscopy, are presented. The interpretation is based on the results of Hartree-Fock-Slater calculations, using STF basis sets of double zeta quality. Implementation of relativistic corrections to the ionization energies of TeBr_{2} show that off-diagonal matrix-elements of the spin-orbit operator give rise to a splitting in the nonrelativistically almost degenerate Br lone pair orbitals. This splitting was observed experimentally. The assignments find additional support in a comparison with PE results of related dihalides.

Relativistic and nonrelativistic charge deformation densities are calculated for the molecules ZnCl_{2}, CdCl_{2} and HgCl_{2} by using the LCAO HFS method and the relativistic perturbation treatment based on this method. Relativistic effects on the deformation densities are too small to be detected by present experimental techniques in compounds of 1st- and 2nd-row transition metals, but these effects may be observed in 3rd-row transition metal compounds, at least if metal s-orbitals are involved in the bonding.

The origin of the well-established relativistic bond contractions is investigated in the Au_{2}, AuH and AuCl model systems. This contraction is not caused by relativistic orbital contractions. It has to be ascribed to a relaxation of kinetic repulsion, which is quite independent of changes in the form of the orbitals.

Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

Bond lengths of AuH and TlH are calculated using numerical one-center expansion Hartree-Fock wavefunctions, incorporating relativistic effects as a 1st-order perturbation. The resulting relativistic bond-length contractions thus obtained using nonrelativistic wavefunctions are comparable to the full Dirac-Fock ones. This confirms that the orbital and bond length contractions are "parallel" consequences of the mass-velocity term but that the former is not necessary for the latter.

An introduction of the concept of chemical bonding is given without reference to orbitals.

No abstract available

Ab initio MO calculations, using both minimal (STO-3G) and extended (Roos-Siegbahn) basis sets are reported for the systems methanethiol-imidazole, methanethiol-imidazole-formaldehyde, and methanethiol-imidazole-formamide, which, together with a point-change representation of a long a-helix, form models for the active site of papain. It is shown that the large electric field exerted by the helix in the active-site region is responsible for the presence of the essential residues cysteine-25 and histidine-159 in the form of an ion pair RImH^{+}-S^{-}, which is crucial for a recently proposed mechanism for the catalytic action of the enzyme. Also, an explanation is given for the anomalies in measured pK values for these residues. Detailed studies on the (sub)systems show that minimal basis sets lack the flexibility necessary for describing the type of proton transfer involved. Thus, a-helixes are essential parts of enzymes and they play a significant role in the catalytic process.

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Extended Gaussian basis sets were calculated for the neutral and trivalent rare earth atoms. The characteristics of these sets, which are available on request, are reported. The quality of the wave functions is discussed on the basis of a comparison of a number of calculated properties with those of Hartree-Fock calculations. The agreement shown is very satisfactory.

Theoretical Chemistry, Materials Science Centre, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The dipole, quadrupole, and octupole moments, diamagnetic susceptibility, second moments and second moment anisotropies of charge distribution, electric field and gradient, diamagnetic shielding, and nuclear quadrupole coupling constant of the title molecules are determined by ab initio SCF calculations. The effects of Gaussian basis set size, including d-type polarization functions, and contraction schemes on some of the properties are determined for COF_{2}. The results agree with experiment.

A new formalism is proposed for incorporating solvent effects into the quantum mechanical description of molecular electronic states. In contrast to existing methods, it does not lead to a non-linear effective Hamiltonian, while both the solvent-solvent and the solute-solvent interactions are treated self-consistently. It also accounts more accurately for the solute's electric field than the usual dipole approximationn. Although formally treated on the Hartree-Fock level, the method incorporates dispersion interaction between solute and solvent.

Ab initio SCF MO calculations were performed on the system methanethiol/imidazole/HCHO (modeling the active site of papain) using a rather large basis of Gaussian-type functions. A point charge representation of the long central a-helix present in the enzyme was added to establish the influence of the electric field of the helix (which amounts to 109 V/m in the active site region) on the equilibrium: RSH...Im to and from RS^{-}...ImH^{+}, which is an essential step in a recently proposed mechanism for the catalytic action of papain. The helix stabilizes the ion-pair by 15 kcal/mol more than the neutral form, making the 2 configurations energetically equivalent, and lowers the energy barrier in the reaction path by 8 kcal/mol, thus shifting the equilibrium considerably towards the ionic situation and increasing the rate of proton transfer by several orders of magnitude. Thus, the active site helixes, present in many enzymes, play a pertinent role in enzyme catalysis.

The methodological problems involved in electronic structure determinations of compounds containing heavy elements by the Hartree-Fock-Slater scheme were investigated. The effect of the inner electrons can be simulated by a so called pseudopotential, so that only the valence electrons have to be treated explicitly which constitutes a considerable reduction of computation time. A pseudopotential calculation is able to achieve an accuracy that is comparable to the results of a calculation including the core.

The perturbative treatment of relativistic effects proposed by S. and B. (1979) is extended to molecular systems within the framework of the analytic Hartree-Fock-Slater method of B. and R. (1975) using a numerical integral evaluation scheme. The theory is used to calculate the photoelectron spectra of I_{2} and HgI_{2}.

D labeling showed that [MeS:CH_{2}]^{+} (I) and [MeCH:SH]^{+} (II), formed by loss of Me.bul. from [MeSEt].bul.^{+}, fragment to give HCDH_{f}. A potential energy diagram rationalizing the isomerizations and the principal fragmentation reaction is presented.

Theoretical Chemistry, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

A single STO basis set, consisting of 9 1s and 6 2p functions, was used to calculate matrix Hartree-Fock ground-state energies for several light atoms. The resulting energies are compared with the most accurate existing calculations of these energies obtained by using different basis sets individually optimized for each atom.

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The influence of the inclusion of a reaction field on molecular dynamics simulations of liquid H_{2}O was evaluated, both for a momentary and a delayed reaction field. The influence on radial distribution function and energy is not large, but the spatial dipolar correlation is changed considerably. Total dipole moment fluctuations are much enhanced by the reaction field. If the delay of the reaction field is not taken into account, the reaction field effects are overestimated. Dynamic effects are sensitive to the reaction field: the diffusion constant almost doubles and rotational correlation times decrease. Computer simulations were made over a total time of 55 ps with a cut-off radius of 0.578 nm and a time of 0.002 ps. The small cut-off and large time step do not influence the accuracy of statistical averages generated by the dynamics run.

Small-basis-set SCF-LCAO-MO calculations were made for (C_{5}H_{5})M(C_{7}H_{7}) (M = Ti, V, and Cr) with different basis sets on the free atoms, the free rings, and the moieties (C_{5}H_{5})Ti and (C_{5}H_{5})Cr. The near-minimal basis-set results provide a reasonable basis for discussion of the trends in the charge distributions of the mixed sandwich compounds Mulliken's population analysis shows that the negative charge on the C_{7}H_{7} ring decreases in the order Ti > V > Cr, while the negative charge on the C_{5}H_{5} ring increases in the same order. The observed trends in metal 2p and 3s and C 1s ionization energies are well reproduced by the corresponding ground-state orbital energies. Independent SCF calculations on several electronic configurations of the positive ions show large reorganizations of the charge distributions. The corresponding relaxation energies depend strongly on the metallic character of the ion ground-state orbitals. The trends in the calculated ionization energies only partially agree with those derived from photoelectron spectra.

Theoretical Chemistry, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

The concept of a universal basis set for electronic structure calculation is illustrated by presenting results obtained when basis sets are transferred from one atom to another. A single Slater-orbital basis set, consisting of nine 1s and six 2p functions, produces Hartree-Fock total energies and orbital energies in good agreement with the most accurate calculations of these energies obtained by using different basis sets individually optimized for each atom. Transferability of integrals is a natural consequence of the use of the same basis set for each atom in a molecule.

The electronic structure of the [CuCl_{4}]^{3-} cluster in solid CuCl was calculated using an ab initio Hartree-Fock SCF-MO method. The chemical bonding was discussed using a population analysis of the ground state orbitals. Calculated ionization potentials show the Cu 3d electrons to be less tightly bound than the Cl 3p electrons in agreement with the photoelectron spectrum of CuCl (Goldmann, A.; et al. 1974) and in contrast to predictions using Koopmans' theorem. The high degree of 3d hole orbital localization conflicts with estimates of d-bond covalency. The potential surface for a Cu^{+} ion, moving in the field of neighboring Cl^{-} ions, was determined for the [CuCl_{2}]^{3-} cluster with the Cu^{+} ion displaced towards a face, an edge or a vertex of the tetrahedron. The vibration frequency and activation energy for Cu^{+} diffusion obtained agreed with experimental results.

A perturbation theory for the calculation of relativistic effects on the electronic structure of atoms was developed which treats 1st-order terms self-consistently. The theory was used to calculate the orbital energies and the 1st-order changes in the wave functions of some 5th-row elements and the results were compared with full relativistic Dirac-Fock-Slater calculations.

Binding of PO_{4}^{3-} moieties at N-termini of a-helixes in proteins corresponded with an optimal interaction of the helix dipole (which runs from the C- to the N-terminal) and the charged PO_{4}^{3-}. Studies on 4 enzymes in which the active site is located at the N-terminus of an a-helix (papain, subtilisin, rhodanese, and glyceraldehyde phosphate dehydrogenase) suggested that the helix dipole is used in catalysis. Thus the electric field generated by the backbone of a protein molecule, and by the helixes in particular, is a significant factor which must be included in the discussion of the properties of proteins.

Ab initio self-consistent-field molecular-orbital calculations were performed on octahedral FeF_{6} and Fe(CN)_{6} clusters using extensive basis sets of Gaussian-type functions. Two distances relevant for ferrous and ferric compounds are considered. The results are reported relevant for a determination of the isomer-shift calibration constant for Fe. Good overall consistency with available ^{57}Fe Moessbauer data is found resulting in a value of a_{HF} = (-0.30 0.03)a03 mm/sec for the calibration constant to be used in conjuction with densities on the nucleus calculated in the spin- and symmetry-restricted Hartree-Fock approximation. This value is compared with previous estimatess, a number of which can be corrected on the basis of the present work and are then shown to agree with the present results. Recent attempts to obtain quantitative relativistic corrections by solving the Fock-Dirac equations for Fe and its ions are discussed. A value for the calibration constant appropriate to densities calculated by this method of a_{FD} = (-0.22 0.02)a03 mm/sec is tentatively derived.

In this report a brief survey is presented of some recent work carried out in our group in the field of ab initio calculations on the electronic structure of transition metal and rare earth ions and their compounds. More particularly, the results of calculations on the "crystal field" or "ligand field" configurations and states of the complex ions [FeX_{6}]^{n-} where X = F, CN and n = 3,4, will be discussed. This discussion is followed by a report on accurate Gaussian basis sets determined for the rare earth ions and atoms and some remarks concerning relativistic effects and their relevance for calculations carried out on [EuO_{6}]^{9-}.

Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands

The lowest singlet excited states of the VO_{4}^{3-} complex were calculated as a function of the V-O bond distance using the Hartree-Fock-Slater discrete variational method. The calculated average singlet transition energies ^{1}DE(y1 to 2e) support assignments made before.

The applicability of average-of-configuration wavefunctions within a restricted Fock-Dirac formalism is investigated. Lande factors, calculated from average intermediate state functions show good agreement with experimentally observed ones to states close to Russell-Saunders coupling.

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A nonparametrized pseudopotential method is proposed, involving the Hartree-Fock-Slater model core-valence exchange which is a suitable form for separating core and valence electrons. Approximations in the scheme are tested and the results for CO, Cl_{2}, and Ni_{2} are compared with frozen-core Hartree-Fock-Slater results. Implications for current Hartree-Fock pseudopotential schemes are discussed.

Results of relativistic and non-relativistic single configurational SCF calculations on the lower electronic states of Pm^{3+} are reported. Comparison with experimental data shows that the relativistic scheme based on a single j-j coupled configuration is inadequate to evaluate the effect of spin-orbit interaction.

The charge distribution and the ligand field splitting in the tetrachloro complexes CuCl_{4}^{2-} were investigated by means of the restricted Hartree-Fock method. A rather large basis set of contracted Gaussian type orbitals was employed. The charge distributions was analyzed by means of a Mulliken population analysis. The ligand field splitting 10Dq was compared with literature results known for the octahedral cluster NiF_{6}^{4-} occurring in KNiF_{3}. A detailed analysis was carried out for CuCl_{4}^{2-}. From calculations on a selected number of states of NiCl_{4}^{2-}, the Racah parameters B and C were obtained.

The optical absorption and magnetic CD (MCD) spectra of the mononegative ion of naphthalene in a solution of 2-methyltetrahydrofuran were measured at room temperature at 25,000-41,000 cm^{-1}. Experimental values of the MCD parameter B/D were compared with theoretical data obtained by means of an LCAO SCF CI calculation according to Pariser, Parr and Pople. The agreement between theory and experiment is rather good.

Ab initio molecular orbital calculations using a contracted basis of gaussian orbitals on the system methanethiol/imidazole (Im) are reported. For the H bond S---H---N in this system, which was chosen as a model for the active site of papain(II), a double-well potential was found at a S-N sepn. of 3.35 . This sustains a newly proposed mechanism for the catalytic action of II which involves the existence of an ion pair ImH^{+}-S^{-}. From minimal basis set calculations on the system methanethiol/imidazole/formaldehyde an estimate is obtained for the stabilization of the 1st model by the H bond between imidazole and formaldehyde. In the later calculations, the total energy as a function of proton positions did not show double-well character. The stabilization energy tends to reinforce the shape of the potential as obtained in the former calculations. It is concluded, however, that minimal basis set calculations are inadequate to elucidate the type of mechanism dealt with here.

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The topics reviewed with 28 references include: the ionic model and calculation of the cohesion energies of ion arrangements; crystal-field theory; quant. interpretations of the octahedral ligand-field-splitting parameter (10 Dq); and Racah parameters.

Theoretical Chemistry, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

By using the independent pair-potential approximation (IPPA) (M., 1974), the valence-shell correlation effects in BH were calculated at 9 internuclear separations. The results are compared with several other methods, including the coupled electron-pair approximation (CEPA) and a full CI expansion. The stability of the IPPA against a unitary transformation of the occupied orbitals was also investigated, the IPPA is nearly invariant against such transformations. The procedure of J. L. Dunham (1932) was applied to the results, and the spectroscopic constants obtained from the various approximations are compared. Many of the defects present in the Hartree-Fock part of the potential curve and arising from the use of medium-quality basis sets can be eliminated by combining the correlation results with Hartree-Fock results from good-quality basis sets and reapplying the Dunham procedure. Finally, the IPPA was applied to BH^{+}, and the 1st vertical ionization potential of BH was determined.

The tricyclohexane I with (NC)_{2}C:C(CN)_{2} in CHCl_{3} gave the 1,4-cycloadduct at room temperature instantaneously. II with (NC)_{2}C:C(CN)_{2} in CHCl_{3} gave the corresponding 1,4-cycloadduct after reflux in CHCl_{3} for several hours. The difference in reactivity was explained by ab initio calculations on the model compounds III-VI, bicyclobutane, cyclobutane, butadiene, and cis-but-2-ene.

Ab initio SCF MO calculations with a contracted double-zeta basis set of 168 Gaussian-type functions were performed on TCNQ^{+}, TCNQ, TCNQ^{-}, and TCNQ^{2-}. The ionization potentials obtained from total energy differences are generally 0.25-0.50 eV higher than the corresponding negative orbital energies from the TCNQ calculation and in satisfactory agreement with experimental results. The energy of the disproportionatioin reaction 2TCNQ^{-} to TCNQ + TCNQ^{2-} is calculated to be 4.2 eV. The charge distributions as measured by the gross atomic populations generally deviate from those obtained in earlier p-electron calculations as a consequence of taking the s-electron distribution into account. The atomic charges agree with the limited experimental data available.

De mogelijkheden om het gedrag van kernen en elektronen op atomair niveau te beschrijven, zijn door de invoering van de computer sterk toegenomen. Dit artikel geeft een schematisch overzicht van deze mogelijkheden.

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A method is given for obtaining the common molecular integrals over generalized gaussian functions. The present algorithms are expected to be more efficient than those given by V. et al. (1971, 1972).

Results of restricted SCF calculations on the closed shell ground-state and open shell n-p* excited states of p-benzoquinone will be presented and discussed. The basis set for C end O consisted of 6/3 sets of primitive gaussian orbitals contracted to 4/2 sets. For H 2 Gaussians contracted from 3 primitive ones were employed. In the groundstate the oxygen 'n' orbitals can be described in two equivalent ways: delocalized m.o.'s of odd and even symmetry or m.o.'s localized essentially on each oxygen. In the excited state these two descriptions are no longer equivalent. The SCF excitation energies calculated for the two situations differ drastically, the localized results being some 2.5 eV less than the delocalized ones and in much better agreement with experiment.

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FHSO_{2}-SbF_{5} solutions of the (CMe)_{6}^{2+} dication I, prepared from the diol II, the epoxide III, or hexamethyl-Dewar-benzene, have no UV absorption with a molar extinction >100. This was confirmed by SCF calculations on the electronic spectrum of the parent nonclassical cation (CH)_{6}^{2+} (IV). The calculations support the pyramidal nonclassical structure for I.

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Closed shell SCF-MO calculations on (CH)_{6}^{2+} showed that the nonclassical structure (I, R = H) was 25 kcal/mole more stable than the classical structure (II). Calculated effective charges on the 5-membered ring of I (R = H) were consistent with experimental findings for I (R = Me).

The initio MO calculations and spectroscopic and x-ray data were presented to prove that aroatic systems are not planar, but flexible as X-ray data for [2.2]paracyclophane, benzo[c]phenanthrene, corrannulene, and helicenes showed that the molecules were nonplanar. MO calculations on C_{6}H_{6} and C_{10}H_{8} bending along the 1,4-axis and the 9,10 bond, resp., showed an average energy of -0.6 kcal/mole per 5 degree deformation.

Computer programs were developed for applying R. K. Nesbet's method (1968-70) of generalized Bethe-Goldstone equations to the calculation of electron-correlation effects in molecules. The 2nd-, 3rd, and 4th-order contributions to the correlation energy of the valence shells of H_{2}O and of BH, the SCF energy of BH, and the spectroscopic constants of BH were calculated.

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Restricted Hartree-Fock MO calculations were carried out for various states of the cluster NiF_{6}^{4-} "in vacuo" and in a surrounding of several sets of point charges representing the perovskite lattice KNiF_{3}. All electrons were included. A "double-zeta" basis set of contracted Gaussian orbitals was used. The calculations were performed with the computer program IBMOL IV. The Hartree-Fock approximation gives a reasonable description of the covalency effects and the spectral properties of KNiF_{3}. The calculated hyperfine-field parameters fs and fs and the 10Dq value are approximately 10-25% smaller than the experimental values. Similar deviations were found for the spectral transition energies. Magnitude and sign of the crystal-field splitting in this compound can be understood in terms of the well-known ionic electrostatic model provided the Born repulsion is properly taken into account.

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A method is given for the evaluation of molecular integrals (1-electron, overlap, kinetic energy, dipole moment, nuclear attraction energy, and 2-electron integrals) over generalized Gaussian functions, which allows for relatively simple interpretation of the orbital exponential parameters for molecules of any shape, especially when the orbitals are used directly as localized MO's. Orbitals of s and p symmetry are treated.

The relation proposed by J. Linderberg (1967) for the resonance integral (b), appearing in p-electron MO theory, and the derivative (dS/dR) of the overlap integral with respect to the internuclear distance cannot be applied in complete-neglect-of- differential-overlap (CNDO) theory. The corresponding relation in Pople-Pariser-Parr theory, which gives correct values for b, can be used to simplify the expressions appearing in the treatment of ORD and CD spectra. The relation between the exact and approximate values of dS/dR is shown for various hydrides (e.g., LiH, CH, SH, FH). Orbital energies, dipole moments, and net charges on the F atom in ClF obtained from full SCF and CNDO calculations are compared.

Rekenmethoden gericht op een zo goed mogelijke benadering van de oplossingen van de Schrödinger vergelijking voor electronen in moleculen vinden meer en meer toepassing als hulpmiddel bij het chemisch-fysisch onderzoek. Dit artikel beoogt een indruk te geven van de achtergrond en betekenis van deze methoden en van de reken- en computerproblemen waarmee ze gepaard gaan.

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Results of ab initio calculations on the crystal field splitting in KNiF_{3} are presented and discussed. Magnitude and sign of these splittings can be understood in terms of the ionic model provided that the Born repulsion terms are properly taken into account.

The use of ellipsoidal Gaussian type orbitals in ab initio calculations on molecular systems of small and intermediate size is demonstrated, in both nonlinear and SCF MO schemes. The method is an extension of Frost's Floating Gaussian Orbital Method. Results for conformational properties (barriers to internal rotation in ethane and 1,3-butadiene) are better than those obtained with basis sets containing only spherical Gaussians. The usefulness of very small basis sets is discussed.

A simple computational scheme is given for nonempirical calculations on polyatomic molecules. The "1 Gaussian per electron" basis contains ellipsoidal Gaussian orbitals for the valence electrons. Geometries of the 1st-row hydrides are predicted quantitatively. Comparison with accurate computations shows that the description of the valence electrons is essentially correct.

The presence of intensity alternations in C_{2} Swan emission spectra of flames is analyzed. The nonequilibrium rotational energy distribution is caused by collision-induced electronic transitions between the radiating A ^{3}P_{g} state and the vibrationally excited A' ^{3}S_{g}^{-} or A" ^{3}S_{u}^{+} states of C_{2}. The selection rules are derived on the basis of the parity properties of the relevant states and the various terms of the multiple expanded interaction potential. The selection rules lead to an unambiguous choice between the collision-induced transitions A" ^{3}S_{g}^{+} to A ^{3}P_{g} or A ^{3}P_{g} to A' ^{3}S_{g}^{-}. On the basis of available experimental information on the pressure dependence of the effect, additional evidence is given that a formation process is responsible for the occurrence of the intensity alternations. Examples of intensity alternations in Si_{2} and N_{2} are briefly discussed.

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The photochromic behavior of the title compd. (I) was studied by E.S.R. measurements. After UV irradiating a powder of I at 77 K for 20 min., a triplet E.S.R. spectrum was observed. The spectrum could be reproduced by computer simulation by using an anisotropic g-tensor (g_{xx} = 2.00950, g_{yy} = 2.00280, and g_{zz} = 2.00232) and zero-field splitting parameters D' and E' of 99 and 2.3 gauss, resp. The triplet spectrum arose from the magnetic coupling between a naphthoxy radical and a Cl atom, while the central weak lines were due to the interaction between 2 naphthoxyl radicals.

The reliability of the results for the calculation of cohesion energies by using the pure ionic model (M. D. Tosi, 1964) for compounds that are not purely ionic is discussed. If the distance R_{x} for which the Coulomb energy of the pair M^{+}X^{-} equals the energy of the neutral atoms MX, is greater than the critical distance R_{cr}, the wavefunction will change its character at the distance from atomic to ionic change with no or almost no energy change, and the ionic model gives practically the exact cohesion energy for distances down to R = R_{cr}. For shorter distances short range quantum mechanical effects occur, resulting in kinetic and potential energy changes which roughly counteract each other.

For an experimental study of the influence of activator center symmetry on spectra, the following compounds with varying site symmetry of Eu^{3+} were chosen from among mixed metal oxides (1) strict center of symmetry (O_{h}), Ba_{2}GdNbO_{6}; (2) no center of symmetry (D_{3}), YA1_{3}B_{4}O_{12}; and (3) small deviations from inversion symmetry, a number of compounds. As a spectroscopic characteristic on intensity ratio was chosen: [^{5}D_{0} -^{7}F_{2} (elec. dipole)]/[^{5}D_{0} - ^{7}F_{1} (magnetic dipole)] = 0.7 for case (1), 7 for case (2), and 0.3-7 for case (3). This confirms qualitatively the principles of the theories of intensities of rare-earth-ion emission.

The absorbed energy transformations within an activator center (Eu^{3+}) is investigated by using a great no. of polycryst. luminophors at cathode and uv excitations. Kinetics of luminescence from ^{5}D_{0} and ^{5}D_{1} showed that the population of ^{5}D_{0} proceeds from ^{5}D_{1} or is directly due to radiationless transitions from higher levels. In some luminophors (e.g. LaInO_{3}) this process is dominant. Decay times are given for Gd_{2}O_{3}-Eu, Y_{2}O_{3}-Eu, YVO_{4}-Eu, SrTiO_{3}-Eu, LaCl_{3}-Eu, GdBO_{3}-Eu, EuCl_{3}.6D_{2}O, EuCl_{3}.6H_{2}O, and Eu_{2}(SO_{4})_{3}.8H_{2}O and are 0.680-2.5 for the ^{5}D_{0} line and 0.011-0.550 msec. for the ^{5}D_{1} line. The phosphors were prepd. by solid-state reactions and were irradiated by a pulses cathode-ray beam in a continuously evacuated tube in which the phosphors can be easily exchanged. The pulse length could be varied from about 0.5 .mu.sec. to several millisec.; the repeating frequency could be varied 16 Hz.-200 kHz. A grating monochromator was placed in front of the window of the tube in order to study the rise and decay of the various lines.

Experimental and theoretical arguments against the ionogenic bond model are discussed. The model is consistent with the virial theorem of the quantum mechanics, but does not give a good picture of the distribution of charge in a crystal. Near the equilibrium distance R_{e} of the ions in a crystal M^{+}X^{-}, the rapid increase of the repulsion implies a great change in kinetic energy, which must cause a change in the distribution of charge, unaccounted for in the classic hard sphere ionic model. The model gives reliable calculated values for the lattice energy of the crystal, until R < R_{kr} when short range effects are noticeable and lead to an energy higher than calculated from the Madelung term, but the error remains small (R = distance M^{+}X^{-} R_{kr} = distance to which the Madelung energy gives the mutual-potential energy accurate to within 1%). When R_{e} < R_{x} < R_{kr} (R_{x} is the distance where the Coulomb energy of M^{+}X^{-} = energy of the neutral atoms M + X), the calculated Madelung energy at R_{e} can be higher or lower than or equal to the experimental energy. A table of molecular and crystal bonding energies of 35 mostly diatomic compounds shows that the model is applicable to some covalent- and metal-type bondings, with exception of CCl_{4} where R_{x} < R_{e}.

The dependence of the relative intensities of the ^{5}D_{0} to ^{7}F_{1} and ^{5}D_{0} to ^{7}F_{2} fluorescent transitions of Eu^{3+} in metal oxide host lattices on the presence or absence of a center of symmetry is discussed and illustrated by a few examples. The occurrence of the ^{5}D_{0} to ^{7}F_{0} transition in lattices where the Eu^{3+} site symmetry is C_{n}, C_{s}, or C_{nv} is noted. Its intensity, which can be quite high in some cases, is explained by allowing a linear term to enter the usual crystal field expansion.

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II. Inner Shell 4p and 4d Atomic Orbital

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I. Inner Shell 3d and 4s Atomic Orbital

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