Spin-orbit multireference multistate perturbation theory

A Hilbert-space multireference perturbation theory is presented to treat spin-dependent operators, and specifically formalism is derived for spin-orbit coupling. The zeroth-order states are obtained by diagonalizing the spin-free portion of the full Hamiltonian within a complete active space (CAS). The perturbation contains an additional spin-dependent term. A subset of the CAS self-consistent-field (CASSCF) states are permitted to mix via an effective Hamiltonian that is computed through the second order. The efficiency of the theory in capturing electron correlation and relativistic contributions is tested on a variety of systems, including atoms (C,Si,Ge,Os) and diatomic molecules (NiCl, PdCl, PtCl). The results are found to be very satisfactory. For example, the Os results reproduce the experimental results for the lowest levels within 1000 cm^-1 and consistently good results for higher excited states.