Natural Orbital Non-Orthogonal Configuration Interaction

TL;DR

This paper introduces a systematic and compact non-orthogonal configuration interaction (NOCI) method that treats multiple electronic states equally, addressing key challenges in constructing accurate state bases.

Contribution

A novel NOCI approach that is systematic, compact, and capable of treating all electronic states on equal footing, validated through simple model systems.

Findings

Effective in constructing compact NOCI expansions

Treats multiple electronic states on equal footing

Validated on simple model systems

Abstract

Non-orthogonal configuration interaction (NOCI) is a generalization of the standard orthogonal configuration interaction (CI) method and offers a highly flexible framework for describing ground and excited electronic states. However, this flexibility also comes with challenges, as there is still no clear or generally accepted approach for constructing a compact and accurate state basis for NOCI. In this work, we take a step toward addressing this challenge by introducing a novel NOCI approach designed with three primary objectives: (1) ensuring the method is systematic, (2) achieving a compact NOCI expansion, and (3) treating all electronic states of interest on equal footing. The development of our approach is presented step by step, with each building block evaluated and validated through applications to simple model systems, demonstrating its effectiveness and potential.