Spin-orbit splittings in the third-row transition elements: Comparison of effective nuclear charge and full Breit-Pauli calculations

The spin-orbit splittings of low-lying states in third-row transition eleme nts were calculated using both an effective core potential (ECP) method wit hin the one-electron (Z(eff)) approximation and all-electron (AE) methods u sing three different approaches. The wave functions were obtained using the multiconfiguration self consistent field (MCSCF) method followed by second -order configuration interaction (SOCI) calculations. All calculated result s, except for the ones on atomic Ir, are in reasonable agreement with the c orresponding experimental observations. The unsatisfactory results for atom ic Ir are attributed to the poor theoretical prediction of the adiabatic en ergy gap between the lowest two IF states. This gap has an incorrect sign i n AE calculations without scalar relativistic corrections, but the gap can be reproduced qualitatively if these corrections are added using the newly developed RESC (relativistic elimination of small components) scheme. As a result, the AE calculations with the RESC approximation give spin-orbit spl ittings similar to those obtained by the ECP calculations with the Z(eff) a pproximation.