Spin-orbit coupling in molecules: chemistry beyond the adiabatic approximation

An extensive introduction to spin-orbit coupling (SOC) is presented, starting from a discussion of the phenomenological operators and general chemical importance of SOC to studies of chemical reactions. Quantitative SOC operators are discussed, and the symmetry properties of the SOC Hamiltonian important for understanding the general features of SOC are summarized. Comparison of the one- and two-electron contributions to SOC is given, followed by a discussion of commonly used approximations for the two-electron part. Applications of SOC to studies using effective and model core potentials have been analysed. The theoretical discussion is illustrated with numerous practical examples, including diatomic molecules (with an emphasis on hydrides) and some examples for polyatomic molecules. The fine structure of the SOC interaction (vibrational dependence) for some diatomic molecules has been elucidated.