Theoretical treatment of the spin–orbit coupling in the rare gas oxides NeO, ArO, KrO, and XeO

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Abstract
Ab initio configuration‐interaction calculations have been carried out on the six electronic states of RgO (Rg=Ne, Ar, Kr, and Xe) arising from the Rg(1S)+O(3P, 1D, 1S) separated atom limits similar to the calculations of Dunning and Hay [J. Chem. Phys. 66, 3767 (1977)]. The degree of charge transfer and, hence, the extent to which the interactions are attractive is observed to be sensitive to the method of configuration selection. The off‐diagonal spin–orbit coupling matrix elements 〈3Π‖Hso‖1Σ+〉 and 〈3Σ−‖Hso‖1Σ+〉 were calculated for the lowest 3Π, 3Σ−, and 1Σ+ states of RgO as a function of internuclear distance using the configuration‐interaction wave functions and the full microscopic spin–orbit Hamiltonian. All one‐ and two‐electron integrals were included. These results are compared in detail with the spin–orbit matrix elements of Cohen, Wadt, and Hay [J. Chem. Phys. 71, 2955 (1979)] computed using equivalent wave functions and an effective spin–orbit operator composed only of one‐electron, one‐center terms. The good agreement between the results strongly supports the use of an effective spin–orbit operator of this type. The success of their model results from the fact that electron interaction effects can be adequately accounted for by a single adjustable parameter that can be determined from the atomic fine structure. Apparently for the RgO molecules, the numerical results are a more sensitive test of the wave functions (particularly to the extent of charge transfer) than to the exact evaluation of all terms in the full spin–orbit operator. In fact, for the heaviest system considered, XeO, the effective operator approach actually appears to be superior to the ab initio treatment.