Understanding the Cavity Born-Oppenheimer Approximation

TL;DR

This paper demonstrates that for realistic coupling strengths, Cavity Born-Oppenheimer energies and spectra can be accurately approximated using standard electronic-structure calculations, elucidating the physical effects behind CBO results.

Contribution

The authors show how to recover CBO energies and spectra from standard calculations, providing a practical alternative to complex CBO methods and clarifying their physical basis.

Findings

CBO energies and spectra can be approximated with high accuracy using out-of-cavity quantities.

The physical effects underlying CBO results are identified and explained.

The methodology offers an efficient alternative to full CBO calculations.

Abstract

Experiments have demonstrated that vibrational strong coupling between molecular vibrations and light modes can significantly change molecular properties, such as ground-state reactivity. Theoretical studies towards the origin of this exciting observation can roughly be divided in two categories, with studies based on Hamiltonians that simply couple a molecule to a cavity mode via its ground-state dipole moment on the one hand, and on the other hand ab initio calculations that self-consistently include the effect of the cavity mode on the electronic ground state within the cavity Born-Oppenheimer (CBO) approximation; these approaches are not equivalent. The CBO approach is more rigorous, but unfortunately it requires the rewriting of electronic-structure code, and gives little physical insight. In this work, we exploit the relation between the two approaches and demonstrate on a real molecule (hydrogen fluoride) that for realistic coupling strengths, we can recover CBO energies and spectra to high accuracy using only out-of-cavity quantities from standard electronic-structure calculations. In doing so, we discover what the physical effects underlying the CBO results are. Our methodology can aid in incorporating more, possibly important features in models, play a pivotal role in demystifying CBO results and provide a practical and efficient alternative to full CBO calculations.