The ab initio study of molecules and crystals that contain heavy elements is a growing field of research. Whereas most of the properties of light molecules can be satisfactory explained in a non-relativistic quantum mechanical model, some of the properties, such as the pair of 2D lines in the spectrum of the sodium atom which are split due to spin-orbit interactions, can only be explained by relativistic influences. If one wants to study molecules or crystals with elements in the lower regions of the periodic table then one has to consider relativistic effects by introducing relativistic corrections or more thoroughly by using a relativistic model. For studies with a relativistic model one may use the program package MOLFDIR.
P.J.C. Aerts started the development of the package in 1981 and produced the programs MOLFDIR, RELONEL, RELTWEL and MFDSCF (in a closed shell version). His successor, O. Visser, adapted the MFDSCF program to an open shell version using the average of configuration energy expression. He also constructed the programs ROTRAN and GOSCIP that allowed him to project out the manifold of open shell states by a small CI calculation using the average spinors to construct a many-electron basis. L. Visscher developed a program, called DIRRCI, to do larger CI calculations. He also constructed the programs PROPAN for analysing the wave function and GENBAS for generating general contracted basis sets. Other contributors are H. Merenga who has speeded up the Dirac-Fock calculations by introducing DIIS and by implementing Quadratic Convergence and W.A. de Jong who has implemented a two open shell formalism and some additional analysing tools in DIRRCI. L. Visscher also added Coupled Cluster (CCSD + triples corrections). SCF and CCSD(T) calculations can also be run in parallel mode using MPI for communication (M.Pernpointner, W.A.de Jong, L.Visscher, R.Broer.)