Applicability of the lowest-order two-electron Breit–Pauli relativistic correction in many-electron heavy and super-heavy elements

The applicability of the lowest-order two-electron Breit–Pauli (2e-BP) relativistic correction incorporated with the infinite-order Foldy–Wouthuysen (IOFW) transformation was examined in heavy and super-heavy elements. Comparing with the non-relativistic Coulomb repulsion term, in cases of moderately heavy noble-gas atoms (He to Ar), the 2e-BP correction gave the SCF energies practically equivalent to those obtained with the Dirac–Fock/Coulomb method. The spin-orbit splittings were also corrected reasonably. On the other hand, in heavy noble-gas atoms (Kr to Rn), the 2e-BP correction tended to overshoot energy lowering, and in a super-heavy atom (E118), evidences of collapse of the 2e-BP correction were observed.