The doubly occupied configuration interaction (DOCI) is a black box method capable of describing strong correlation with a factorial scaling. DOCI energies can be reproduced by a pair coupled cluster doubles (pCCD) approach (or the antisymmetrized product of one-reference orbital geminals) with a more favorable cubic scaling if we neglect orbital optimization. In this work, we describe new seniority zero approximations that offer the same computational cost as pCCD, based on the coupled-electron pair approximation, the averaged coupled-pair functional, averaged quadratic coupled-cluster, and the parametric two-electron reduced density matrix (p2RDM) approach. Among these methods, the pair p2RDM (pp2RDM) provides the best approximation to DOCI energy. While pp2RDM energies are comparable to those obtained from pCCD, its RDMs are simpler to implement and are Hermitian, thus can potentially improve the quality of molecular properties provided by pCCD. Our study also indicates that pp2RDM RDMs are better approximations to DOCI ones than those from pCCD for the systems we investigated.

Seniority zero energy functionals derived from configuration interaction with pair excitations

ACS Fall 2021

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