Relativistic effects in K-shell ionizations: SAC-CI general-R study based on the DK2 Hamiltonian

Relativistic effects in K-shell ionizations were studied using the symmetry adapted cluster-configuration interaction (SAC-CI) general-R method based on the spin-free part of the second-order Douglass-Kroll-Hess (DK2) Hamiltonian. The core-electron binding energies (CEBEs) of F, Si, P, S, and Cl atoms of several molecules were calculated. The relativistic effect in the CEBEs of the second-row atoms was 4-9 eV. The relativistic effect was mostly overestimated by the Koopmans' theorem and was reduced to the extent of 0.15-0.4 eV by including the orbital relaxation and electron correlations. The present method provides a simple and reliable computational tool for calculating the CEBEs of molecules containing heavy elements.