EOM-CC guide to Fock-space travel: the C2 edition

Abstract

Despite their small size, C₂ species pose big challenges to electronic structure methods, owing to extensive electronic degeneracies and multi-configurational wave functions, which lead to a dense manifold of electronic states. We present detailed electronic structure calculations of C₂, C₂⁻, and C₂²⁻, emphasizing spectroscopically relevant properties. We employ the double ionization potential (DIP) and ionization potential (IP) variants of the 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 C₂²⁻, which is metastable with respect to electron detachment.