New implementation of spin‐orbit coupling calculation on multi‐configuration electron correlation theory

Abstract

AbstractFor treating both relativistic effect and electron correlation, the spin‐free exact two‐component and spin‐dependent first‐order Douglas–Kroll–Hess Hamiltonian and the state‐interaction (SI) method are combined to calculate the spin‐orbit coupling (SOC) on multi‐configuration electron correlation theory. In this method, SOC is evaluated via SI over electronic states from the complete active space self‐consistent field calculation, and dynamic correlation can be reckoned with the high‐level multi‐reference electron correlation theory. Work equations to calculate SOC matrix elements over Gelfand states in the framework of the graphic unitary group approach are presented. Benchmark calculations have verified the validity of the implementation. As a pilot application, the internally contracted multi‐reference configuration interaction with the inclusion of SOC via the SI approach produces reasonable equilibrium bond length and harmonic vibrational frequency of the ground state of AuO, as well as the transition energy of .