An approximate method to incorporate spin—orbit effects into calculations using effective core potentials

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An effective one-electron, one-center spin—orbit operator, which has given good results in all-electron calculations, is tested for valence-electron calculations on silicon atom using an effective core potential (ECP). The results are compared to recent calculations by Stevens and Krauss, who used the ab initio spin—orbit operator developed by Ermler et al. and based on Dirac—Fock calculations. Although the latter approach does lead to superior agreement with experiment, the results of the effective one-electron, one-center spin—orbit operator are quite good. In fact, for cases in which the radial character of the valence orbitals does not change drastically, the effective one-electron, one-center spin—orbit operator should provide comparable accuracy to the spin—orbit operator of Ermler et al. The present approach has the advantages of simplicity since it does not require the calculation of relativistic effective potentials from Dirac—Fock wavefunctions.