Combining complex conjugation, time-reversal, and spin symmetry projection of coupled cluster wave functions

TL;DR

This paper introduces a method combining symmetry projection techniques with coupled cluster theory to improve the description of static correlation and restore symmetries like time reversal in molecular wave functions.

Contribution

It develops a novel approach integrating complex conjugation, spin, and time-reversal symmetry projections within coupled cluster theory, enhancing accuracy for strongly correlated systems.

Findings

Achieves good agreement with full configuration interaction results.

Successfully recovers time-reversal symmetry using spin flip projection.

Effectively captures static correlation in molecular systems.

Abstract

Complex conjugation symmetry breaking and restoration generate two non-orthogonal configurations at the Hartree-Fock level that can capture static correlation naturally. In conjunction with broken spin-symmetry coupled cluster theory, the symmetry-projected wave function shows good agreement with full configuration interaction in beryllium hydride insertion, lithium fluoride dissociation, and symmetric stretching of tetrahedral H$_4$. By adding spin flip projection, we can also recover time reversal symmetry in the same coupled cluster framework.