Breit corrections to individual atomic and molecular orbital energies.

Several issues concerning Breit correction to electron-electron interaction in many-electron systems, which are important in precise atomic and molecular calculations, are presented. At first, perturbative versus self-consistent calculations of Breit correction were studied in selected cases. Second, the Z-dependence of Breit contribution per subshell is shown, based on values calculated for selected atoms with 30 ≤ Z ≤ 118. Third, the relations between magnetic and retardation parts of Breit interaction are analyzed. Finally, Gaunt contribution calculated for Kr, Xe, and Rn noble gas atoms and its iso-electronic HBr, HI, and HAt diatomic molecules has been compared to full-Breit atomic calculations. We found that Breit corrections should be treated by self-consistent calculations and that there is a functional dependence of those corrections for subshells as εnl Breit (Z)≃a×Zb . We also found that molecular Gaunt corrections are close to their atomic counterparts for inner electrons though they are not for outer orbitals. In any case, accurate calculations must include retardation correction in addition to Gaunt.