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

S. Gulania, T.-C. Jagau, and A. I. Krylov
Faraday Disc.  217, 514 – 532 (2019)

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.

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