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.
Equation-of-motion Coupled-cluster singles, doubles and(full) triples for doubly ionized and two-electron-attached states: A Computational implementation
Double Electron Attachment and Double Ionization Potential Equation-of-Motion Coupled-Cluster Approaches with Full and Active-Space Treatments of 4-Particle-2-Hole and 4-Hole-2-Particle Excitations and Three-Body Clusters
Frozen Natural Orbitals based Equation-of-motion coupled-cluster singles, doubles and triples for Ionized, Double-Ionized, Electron Attached and Two-Electron Attached states
On the Coupled Cluster Doubles Truncation Variety of Four Electrons
Rensselaer Polytechnic Institute（伦斯勒理工学院）；Max Planck Institute of Molecular Cell Biology and Genetics（马克斯·普朗克细胞生物学与遗传学研究所）；Technische Universität Dresden（德累斯顿工业大学）；Max Planck Institute for Mathematics in the Sciences（马克斯·普朗克科学数学研究所）
