Similarity-transformed equation-of-motion coupled-cluster singles and doubles method with spin-orbit effects for excited states.

The similarity transformed equation-of-motion coupled-cluster method (STEOM-CCSD) for excited states is extended to treat spin-orbit coupling interactions (SOIs) using the spin-orbit mean field approximation of the Breit-Pauli Hamiltonian. Two possible schemes to include the spin-orbit splittings of excited states within the STEOM-CCSD model are formulated. They are identified as "diagonalize-then-perturb" and "perturb-then-diagonalize" approaches. The second approach is more suited for cases where SOI is larger, and the first approach breaks down. With the aid of the standard many-body diagrammatic techniques, expressions for all the necessary matrix elements can be derived. These new formulations are implemented in the ACES III suite of parallel coupled cluster programs, and benchmark studies are performed. Numerical tests for several atoms and molecules show a good agreement of calculated spin-orbit splittings to experiment, while also documenting the numerical differences between the two approaches.