Cavity coupled molecules and solids from finite field DFT

Strong light-matter coupling in optical cavities can alter the dynamics of molecular and material systems resulting in polaritonic excitation spectra and modified reaction pathways. For strongly coupled photon modes close in energy to nuclear vibrations the Cavity Born Oppenheimer Approximation (CBOA) in the context of quantum-electrodynamical density functional theory (QEDFT) has been demonstrated to be an appropriate description of the coupled light-matter system. In this work we show that results equivalent to those from a CBOA functional can be obtained from the response of the molecule(s) or material to electric fields, with the cavity parameters (mode frequency and coupling strength) entering only in a post processing step. This procedure enables properties at a range of cavity frequencies and coupling strengths to be obtained from a single set of electric field calculations. The formalism can be applied using existing techniques for characterizing insulating solids in electric fields enabling first principles modeling of cavity coupled systems in periodic boundary conditions. Results for IR spectra and other properties of cavity coupled molecules and solids will be presented.