EOM-CC Guide for Fock-Space Travel: The C2 Edition

Despite their small size, C₂ species pose a big challenge to electronic structure owing to extensive electronic degeneracies and multi-configurational wave functions leading to a dense manifold of electronic states. We present detailed electronic structure calculations of C₂, C₂^-, and C₂^2- emphasizing spectroscopically relevant properties. We employ double ionization potential (DIP) and ionization potential (IP) variants of equation-of-motion coupled-cluster method with single and double substitutions (EOM-CCSD) and a dianionic reference state. We show that EOM-CCSD is capable of describing multiple interacting states in C₂ and C₂^- in an accurate, robust, and effective way. We also characterize the electronic structure of C₂^2-, which is metastable with respect to electron detachment.