The multi-state CASPT2 spin-orbit method

Abstract: We propose the multi-state complete-active-space second-order perturbation theory spin-orbit method (MS-CASPT2-SO) for electronic structure calculations. It is a two-step spin-orbit coupling method that does not make use of energy shifts and that intrinsically guarantees the correct characters of the small space wave functions that are used to calculate the spin-orbit couplings, in contrast with previous two-step methods.