Theoretical aspects

The many-electron equation to be solved is the Dirac-Coulomb-(Breit) equation for N electrons



Here hi is the one-electron Dirac Hamiltonian for electron i


The inner product sigma-dot-p is taken between the vector of three Pauli spin matrices sigma (x,y,z) and the momentum operator p (px,py,pz), acting on the coordinates of electron i. In our notation 12 is used for a 2x2 unit matrix. The scalar potential psi arises in this approximation from the fixed nuclear framework. The program package uses atomic units in which the speed of light, c, is taken to be 137.0359895. In these units m, the mass of the electron, e, the elementary charge are set to 1.

To define a many-electron Hamiltonian one needs a two-electron operator gij. A correct relativistic two-electron operator cannot be written down in closed form. From the theory of Quantum Electro Dynamics one can derive a series expansion of the complete interaction [1]. The first term is the Coulomb interaction


The next term is the Breit [2] correction. Its magnetic part, the Gaunt [3] operator, is presently implemented in the program package

Here a is the vector of four-component Pauli matrices

With the two-electron operator defined as the Coulomb plus the Gaunt operator

the equations 1 and 2 define our basic physical starting point.

References in Theory

  1. I.P.Grant and H.M.Quiney, Adv.At.Mol. Phys. 23, 37 (1988).
  2. G.Breit, Phys.Rev. 34, 553 (1929).
  3. J.A.Gaunt, Proc.Roy.Soc. A122, 513 (1929).

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