Pair extended coupled cluster doubles

TL;DR

This paper introduces the pair extended coupled cluster doubles (pECCD) method, which accurately reproduces the DOCI wave function at mean-field cost and performs well even in challenging attractive interaction cases.

Contribution

The paper presents pECCD, a new method that extends pCCD to accurately replicate DOCI energies and wave functions with low computational cost.

Findings

pECCD reproduces DOCI energies accurately.

pECCD closely matches DOCI wave functions.

pECCD remains effective for attractive interactions where pCCD fails.

Abstract

The accurate and efficient description of strongly correlated systems remains an important challenge for computational methods. Doubly occupied configuration interaction (DOCI), in which all electrons are paired and no correlations which break these pairs are permitted, can in many cases provide an accurate account of strong correlations, albeit at combinatorial computational cost. Recently, there has been significant interest in a method we refer to as pair coupled cluster doubles (pCCD), a variant of coupled cluster doubles in which the electrons are paired. This is simply because pCCD provides energies nearly identical to those of DOCI, but at mean-field computational cost (disregarding the cost of the two-electron integral transformation). Here, we introduce the more complete pair extended coupled cluster doubles (pECCD) approach which, like pCCD, has mean-field cost and reproduces DOCI energetically. We show that unlike pCCD, pECCD also reproduces the DOCI wave function with high accuracy. Moreoever, pECCD yields sensible albeit inexact results even for attractive interactions where pCCD breaks down.