Configuration interaction combined with spin-projection for strongly correlated molecular electronic structures

TL;DR

This paper presents ECISD, a method combining spin-projection and configuration interaction to accurately treat static and dynamic correlations in molecular electronic structures, demonstrated on various molecules.

Contribution

It introduces a novel ECISD approach with Wick theorem for nonorthogonal determinants, improving correlation treatment in quantum chemistry.

Findings

Accurate potential curves for HF, H2O, N2, and hydrogen lattice.

Efficient implementation with direct CI scheme.

Balanced treatment of static and dynamic correlations.

Abstract

We introduce single and double particle-hole excitations in the recently revived spin-projected Hartree-Fock. Our motivation is to treat static correlation with spin-projection and recover the residual correlation, mostly dynamic in nature, with simple configuration interaction (CI). To this end, we present the Wick theorem for nonorhtogonal determinants, which enables an efficient implementation in conjunction with the direct CI scheme. The proposed approach, termed ECISD, achieves a balanced treatment between dynamic and static correlations. To approximately account for the quadruple excitations, we also modify the well-known Davidson correction. We report our approaches yield surprisingly accurate potential curves for HF, H2O, N2, and a hydrogen lattice, compared to traditional single reference wave function methods at the same computational scaling as regular CI.