Cavity-Born Oppenheimer Approximation for Molecules and Materials via Electric Field Response

TL;DR

This paper introduces an ab initio method using the cavity Born-Oppenheimer approximation to efficiently compute vibro-polariton and phonon-polariton spectra in molecules and materials coupled to optical cavities, leveraging standard density functional perturbation theory.

Contribution

The method enables calculation of cavity-coupled spectra using existing electric response properties without additional electronic structure calculations, simplifying analysis of cavity effects.

Findings

Efficient spectra computation across various cavity parameters.

Spectra interpretation via cavity-independent electric response properties.

Application to 2D insulators and molecular systems.

Abstract

We present an ab initio method for computing vibro-polariton and phonon-polariton spectra of molecules and solids coupled to the photon modes of optical cavities. We demonstrate that if interactions of cavity photon modes with both nuclear and electronic degrees of freedom are treated on the level of the cavity Born-Oppenheimer approximation (CBOA), spectra can be expressed in terms of the matter response to electric fields and nuclear displacements which are readily available in standard density functional perturbation theory (DFPT) implementations. In this framework, results over a range of cavity parameters can be obtained without the need for additional electronic structure calculations, enabling efficient calculations on a wide range of parameters. Furthermore, this approach enables results to be more readily interpreted in terms of the more familiar cavity-independent molecular electric field response properties, such as polarizability and Born effective charges which enter into the vibro-polariton calculation. Using corresponding electric field response properties of bulk insulating systems, we are also able to obtain $\Gamma$ point phonon-polariton spectra of two dimensional (2D) insulators. Results for a selection of cavity-coupled molecular and 2D crystal systems are presented to demonstrate the method.