Spin-Symmetry Projected constrained Unrestricted Hartree-Fock

TL;DR

This paper presents a novel spin symmetry restoration method in Hartree-Fock calculations using a non-orthogonal configuration interaction approach, improving accuracy for correlated electron systems.

Contribution

It introduces the SPcUHF method that restores spin symmetry in constrained-UHF using NOCI, reducing computational costs while capturing static and dynamic correlations.

Findings

Successfully applied to 4-, 6-, and 8-electron systems.

Effectively restores spin symmetry in mean-field calculations.

Captures both static and dynamic electron correlations.

Abstract

We introduce an electronic structure approach for spin symmetry breaking and restoration from the mean-field level. The spin-projected constrained-unrestricted Hartree-Fock (SPcUHF) method restores the broken spin symmetry inherent in spin-constrained-UHF determinants by employing a non-orthogonal Configuration Interaction (NOCI) projection method. This method includes all possible configurations in spin space compatible with a Clebsch-Gordon recoupling scheme in a NOCI calculation. The tunable one-pair-at-a-time characteristics of the symmetry-breaking process in c-UHF allow us to reduce the computational costs of full projection. SPcUHF is tested on 4-, 6-, and 8-electron systems that exhibit dominant static and/or dynamic correlations.