Approaching exact hyperpolarizabilities via sum-over-states Monte Carlo configuration interaction

TL;DR

This paper introduces a sum-over-states Monte Carlo configuration interaction method to accurately compute higher-order dipole properties and hyperpolarizabilities, demonstrating its effectiveness on small molecules and atoms with multireference character.

Contribution

The paper presents a novel approach combining sum-over-states calculations with Monte Carlo configuration interaction to efficiently approximate hyperpolarizabilities in complex systems.

Findings

Accurate hyperpolarizability calculations for small molecules.

Extension of the method beyond full configuration interaction results.

Successful modeling of multireference and strongly correlated systems.

Abstract

We propose using sum-over-states calculations with the compact wavefunctions of Monte Carlo configuration interaction to approach accurate values for higher-order dipole properties up to second hyperpolarizabilities in a controlled way. We apply the approach to small systems that can generally be compared with full configuration interaction (FCI) results. We consider hydrogen fluoride with a 6-31g basis and then look at results, including frequency dependent properties, in an aug-cc-pVDZ basis. We extend one calculation beyond FCI by using an aug-cc-pVTZ basis. The properties of an H$_{4}$ molecule with multireference character are calculated in an aug-cc-pVDZ basis. We then investigate this method on a strongly multireference system with a larger FCI space by modelling the properties of carbon monoxide with a stretched geometry. The behavior of the approach with increasing basis size is considered by calculating results for the neon atom using aug-cc-pVDZ to aug-cc-pVQZ. We finally test if the unusual change in polarizability between the first two states of molecular oxygen can be reproduced by this method in a 6-31g basis.