The restricted active space (RAS) state interaction approach with spin–orbit coupling

A method to compute spin–orbit coupling between electronic states is presented. An effective one-electron spin–orbit Hamiltonian is used, based on atomic mean field integrals. The basic electronic states are obtained using the restricted active space (RAS) SCF method. The Hamiltonian matrix is obtained by an extension of the restricted active space state interaction (RASSI) method. Several hundred states can be included. Tests for atoms and molecules from the entire periodic system show accurate results. Computed spin–orbit effects on relative energies are normally accurate within a few percent. The method has been included in the MOLCAS-5.0 quantum chemistry software.