Perturbative calculation of spin-orbit splittings using the equation-of-motion ionization-potential coupled-cluster ansatz.

Spin-orbit splittings for (2)Pi states are calculated within coupled-cluster (CC) theory via first-order degenerate perturbation theory. Using the equation-of-motion CC variant for ionization potentials (EOMIP-CC), the two components of the considered (2)Pi state are treated in a balanced way by generating both radical states via annihilation of one electron out of the CC wave function of the corresponding anion. We report on the implementation of the described approach within the CC singles and doubles approximation. To ensure computational efficiency, an atomic mean-field approximation for the spin-orbit integrals is used, resulting in a formulation in terms of one-electron transition-density matrices. Calculations for XH radicals (X=O, S, Se) lead to satisfactory agreement with experiment. For (2)Pi systems that within an EOMIP-CC treatment can only be reached from a triplet reference state (e.g., CF and O(2) (+)) the influence of spin contamination is found to be negligible.