Correlation and relativistic effects for many-electron systems

Abstract
The N-electron Dirac-Coulomb equation is partitioned into an equivalent of the Schrödinger equation and perturbations. The perturbations involved are ordered according to increasing order of smallness in c−2. The electron-electron interaction itself is also treated as a perturbation. This leads to a new version of the double perturbation theory which provides the basic ideas for simultaneous study of the correlation and relativistic effects. Second-order results for some members of the He isoelectronic sequence are obtained by starting with the Hartree-Fock approximation and using the variational-perturbation method. The total energies calculated through second-order are in excellent agreement with their MCDF counterparts.