The localizability of valence space electron–electron correlations in pair-based coupled cluster models

Abstract

Inspired by the success of very restrictive local active space approaches like Generalized Valence Bond Perfect Pairing (GVB-PP) and Imperfect Pairing (IP), we investigate the localizability of valence correlation in the context of electron pair models. In particular, the role of two-center local excitations in local active space coupled cluster doubles models is examined in the context of a variety of chemical applications. The full two spatial center pairing model is found to recover the majority of the correlation effects missed by GVB-PP and IP, recovering up to 95% of the untruncated valence correlation energy, and could provide an inexpensive reference wave function for the treatment of additional electron–electron correlations. All of these models improve noticeably upon the Hartree–Fock wave function and can be computed for very large systems due to their only cubic computational cost. Their main weakness is a tendency to exhibit symmetry breaking in systems with multiple resonance structures.