A meta-generalized gradient approximation exchange-correlation functional for the electron-photon interaction in strongly coupled light-matter systems

Strong light–matter coupling inside optical picocavities permits the modification of chemical and physical properties without altering the molecular composition. Theoretical modeling of such phenomena within quantum electrodynamical density functional theory (QEDFT) framework necessitates exchange-correlation (XC) approximations that incorporate both electron–electron (ee) and electron–photon (ep) interactions. In this talk, we will present the development of a meta-generalized gradient approximation (meta-GGA) XC functional specifically targeting the electron-photon interaction. This novel epXC functional is built upon a new semilocal polarizability approximation, which draws from the jellium-with-a-gap model, and it can be extended to a “global hybrid” variant that goes beyond an isotropic model. This development enhances the functional hierarchy for QEDFT and provides a practical tool for modeling polaritonic chemistry.  We plan to present validation calculations assessing the fidelity of our new polarizability model as well as the final epXC functional. Furthermore, we will compare the results obtained with our new mGGA epXC approximation and other QED methods—including QED Hartree-Fock and QED coupled cluster—for the modification of the regioselectivity upon bromination of p-nitrobenzene and the dispersion of the energy levels of upper and lower polaritons in simple systems.