Correlation energies for the He isoelectronic sequence with Z = 1–120 from the third-order Douglas–Kroll Hamiltonian

Recently it has been found that the correlation energies (CEs) for He-like ions derived from the four component relativistic Hylleraas configuration interaction decrease rapidly as the nuclear charge increases. This is in contrast to nonrelativistic results, for which the values are almost constant irrespective of the nuclear charge Z. Here, it is shown that the CEs of He-like ions from the third-order Douglas–Kroll Hamiltonian with a large Gaussian type set 33s28p28d28f show a similar trend to the four component relativistic CEs, although the absolute values of the calculated CEs are considerably greater than those of the four component ones.