Open-source implementation of the anti-Hermitian contracted Schr\"odinger equation for electronic ground and excited states

TL;DR

This paper presents an open-source implementation of the anti-Hermitian contracted Schrödinger equation (ACSE) for accurately simulating all-electron correlation in molecules, applicable to ground and excited states across various regimes.

Contribution

The authors introduce a scalable, exact Hamiltonian-based ACSE implementation that improves upon traditional methods for strongly correlated electronic systems.

Findings

Demonstrates good accuracy for main group and transition metal systems

Works effectively in weakly and strongly correlated regimes

Applicable to various basis sets and both ground and excited states

Abstract

Efficient simulation of strongly correlated electrons has become a routine tool in molecular electronic structure theory due to recent advances in approximate configuration interaction (CI) techniques. Nonetheless, the quantitative and predictive description of molecular electronic states remains a significant challenge due to the difficulty of computing all-electron correlation beyond CI. Here, we describe a new open-source implementation of the anti-Hermitian contracted Schr\"odinger equation (ACSE) for use in accurate simulation of all-electron correlation in molecules. In contrast to standard approaches via multireference perturbation theory, the scaling of the ACSE does not depend on the complexity of the strongly correlated reference wavefunction. Furthermore, the ACSE employs the exact electronic Hamiltonian, rather than an approximate perturbative Hamiltonian. Our benchmark results demonstrate good accuracy for main group and transition metal systems, in weakly and strongly correlated regimes, with various basis sets, and for ground and excited states. The results suggest that the ACSE has potential as a scalable and robust technique for simulating all-electron correlation in molecular ground and excited states.