Predicting weak-to-strong light-matter coupling in cavities from first principles

Quantum electrodynamical density functional theory (QEDFT) is a first principles, non-perturbative framework for interactions of quantum matter with quantized electromagnetic fields. QEDFT studies have usually considered lossless cavities. Experimentally accessible cavities, however, exhibit finite photon lifetimes leading to incoherences that dominate dynamics of the light-matter excitations. Here we extend QEDFT by considering lossy cavity modes. This allows us to study ab initio correlated optical interactions in matter ranging from the weak-coupling to strong-coupling regime. As an example, we study excited-state dynamics and spectral responses of benzene and chlorobenzene under weak-to-strong light-matter coupling. By tuning the coupling we achieve cavity-mediated energy transfer between electronic excited states. We interpret the first principles results using a Fano-like model parametrized with the (QE)DFT data. This extension to QEDFT moves toward closing the loop between first principles calculations employed in electronic structure theory and parametric models of the quantum optics community.