Theoretical approaches for treating non-valence correlation-bound anions.

In this work, we use a model (H2 O)4 cluster, the bent CO2 molecule, and tetracyanoethylene as systems to explore the applicability of various electronic structure methods for characterizing non-valence correlation-bound anion states. The methods examined include the algebraic diagrammatic construction, various equation-of-motion coupled cluster methods, orbital-optimized MP2, and Brueckner coupled cluster doubles with perturbative triples. We demonstrate that the key to treating this challenging class of anions is the use of methods that include adequate orbital relaxation in response to long-range dispersion-like correlation effects.