Amplitude determinant coupled cluster with pairwise doubles

TL;DR

This paper introduces a variational coupled-cluster-like method based on amplitude determinants for pairwise doubles, achieving similar accuracy to pCCD and DOCI with polynomial cost and Monte Carlo evaluation.

Contribution

It develops a new variational ansatz based on amplitude determinants for pairwise doubles, combining size-consistency with Monte Carlo methods.

Findings

Performs similarly to pCCD and DOCI in dissociation tests

Remains size-consistent and polynomial in cost

Enables variational treatment via Monte Carlo methods

Abstract

Recently developed pair coupled cluster doubles (pCCD) theory successfully reproduces doubly occupied configuration interaction (DOCI) with mean field cost. However, the projective nature of pCCD makes the method non-variational and thus hard to improve systematically. As a variational alternative, we explore the idea of coupled-cluster-like expansions based on amplitude determinants and develop a specific theory similar to pCCD based on determinants of pairwise doubles. The new ansatz admits a variational treatment through Monte Carlo methods while remaining size-consistent and, crucially, polynomial cost. In the dissociations of LiH, HF, H2O and N2, the method performs very similarly to pCCD and DOCI, suggesting that coupled-cluster-like ansatzes and variational evaluation may not be mutually exclusive.