Treating spin–orbit coupling and spin–spin coupling in the framework of the iterative configuration expansion selected CI

Spin-adapted configuration state functions (CSFs) provide a compact many-electron basis for open-shell molecules. This basis is employed in one flavor of the recently introduced iterative configuration expansion (ICE) selected CI method. In this work, we implemented spin-dependent operators like spin–orbit coupling and direct spin–spin coupling for use in quasidegenerate perturbation theory on top of nonrelativistic/scalar-relativistic ICE wave functions. At the core of the new implementation are matrix elements of spin tensor excitation operators between CSFs, which are evaluated as products of orbital-specific factors. Two applications, the electron paramagnetic resonance g-factors of a MoIII-based catalytic intermediate and the zero-field splitting in dioxygen, illustrate the capabilities of the new method.