# A General Approach for Multireference Ground and Excited States using Non-Orthogonal Configuration Interaction

## TL;DR

This paper introduces a general method combining non-orthogonal configuration interaction with holomorphic Hartree--Fock solutions to accurately describe multireference ground and excited states across various molecular geometries.

## Contribution

It presents a novel protocol using active space SCF metadynamics and complex plane topology to identify and trace multiple Hartree--Fock solutions for multireference state calculations.

## Key findings

- Successfully applied to fluorine dimer dissociation
- Demonstrated effectiveness on cyclobutadiene distortion
- Provides a scalable approach for multireference states

## Abstract

A balanced description of ground and excited states is essential for the description of many chemical processes. However, few methods can handle cases where static correlation is present, and often these scale very unfavourably with system size. Recently, multiple Hartree--Fock (HF) solutions have been proposed as a basis for non-orthogonal configuration interaction (NOCI) to provide multireference ground and excited state energies, although applications across multiple geometries have been limited by the coalescence of HF solutions. Holomorphic HF (h-HF) theory allows solutions to be analytically continued beyond the Coulson--Fischer points at which they vanish but, until now, this has only been demonstrated for small model systems. In this work, we propose a general protocol for computing NOCI ground and excited state energies using multiple HF solutions. To do so, we outline an active space variation of SCF metadynamics that allows a chemically relevant set of HF states to be identified, and describe how these states can be routinely traced across all molecular geometries by exploiting the topology of h-HF solutions in the complex plane. Finally, we illustrate our approach using the dissociation of the fluorine dimer and the pseudo-Jahn--Teller distortion of cyclobutadiene, demonstrating its applicability for multireference ground and excited states.