Spin-orbit splitting for inner-shell 2p states.

A strategy to calculate spin-orbit splitting for inner-shell transitions at an ab initio level is presented. The initial wave function is calculated for a spinless Hamiltonian at a multiconfigurational level, with just a few configurations, followed by multireference configuration interaction in order to establish a set of singlet and triplet states at 2p excitation edge. Then, the full Breit-Pauli Hamiltonian is formed and diagonalized on this state basis. The spin-orbit splitting is determined by a graphical procedure depending on the intensity of the transition from ground state. The specific states studied are those originating from 2p transitions in argon, HCl, H2S, and PH3.