Equation-of-motion coupled-cluster methods for open-shell and electronically excited species: The hitchhiker's guide to Fock space
Equation-of-motion coupled-cluster approach is a versatile electronic structure tool that allows one to describe a variety of multi-configurational wavefunctions within single-reference formalism. This review provides a guide to the established EOM methods illustrated by examples demonstrating which types of target states are currently accessible by EOM. The focus is on applications of EOM-CC to electronically excited and open-shell species. The examples emphasize the advantages of EOM for selected situations, i.e., interacting states of different nature, JT and pseudo-JT states, dense manifolds of ionized states, diradicals, triradicals, etc. The limitations and caveats are also discussed, and practical solutions to some problematic situations are offered. The review also touches upon some formal aspects of the theory and important current developments.