Towards near-exact solutions of molecular electronic structure: Full coupled-cluster reduction with a second-order perturbative correction

TL;DR

This paper presents FCCR(2), an advanced coupled-cluster method with a second-order correction, enabling near-exact electronic structure calculations for complex molecules with strong electron correlation.

Contribution

Introduction of FCCR(2), a novel augmented coupled-cluster method with perturbative correction for improved accuracy in electronic structure calculations.

Findings

Accurately estimates near-exact energies for challenging molecules.

Demonstrates effectiveness on benzene and transition metal complexes.

Shows linear relationship between perturbative correction and total energy.

Abstract

We introduce a new augmented adaptation of the recently developed full coupled-cluster reduction (FCCR) with a second-order perturbative correction, abbreviated as FCCR(2). FCCR is a selected coupled-cluster expansion aimed at optimally reducing the excitation manifold and commutator expansions for high-rank excitations to obtain accurate solutions of the electronic Schrodinger equation in a size-extensive manner. The present FCCR(2) enables the estimation of the residual correlation of FCCR by the second-order perturbative correction $E^{(2)}$ from the complementary space of the FCCR projection manifold. The linear relationship between $E^{(2)}$ and the energy of FCCR(2) allows accurate estimates of near-exact energies for a wide variety of molecules with strong electron correlations. The potential of the method is demonstrated using challenging cases, such as the ground state electronic energy of the benzene molecule in equilibrium and stretched geometries, and the isomerization energy of the transition metal complex, [Cu(NH$_3$)]$_2$O$_2$$^{2+}$.