Ab initio calculations of quantum light-matter interactions in general electromagnetic environments

The emerging field of strongly coupled light-matter systems has drawnsignificant attention in recent years due to the prospect of altering physicaland chemical properties of molecules and materials. Because this emerging fielddraws on ideas from both condensed-matter physics and quantum optics, it hasattracted attention from theoreticians from both fields. While the formeremploy accurate descriptions of the electronic structure of the matter thedescription of the electromagnetic environment is often oversimplified.Contrastingly, the latter often employs sophisticated descriptions of theelectromagnetic environment, while using simple few-level approximations forthe matter. Both approaches are problematic because the oversimplifieddescriptions of the electronic system are incapable of describing effects suchas light-induced structural changes, while the oversimplified descriptions ofthe electromagnetic environments can lead to unphysical predictions because thelight-matter interactions strengths are misrepresented. Here we overcome theseshortcomings and present the first method which can quantitatively describeboth the electronic system and general electromagnetic environments from firstprinciples. We realize this by combining macroscopic QED (MQED) with QuantumElectrodynamical Density-functional Theory. To exemplify this approach, weconsider an absorbing spherical cavity and study the impact of differentparameters of both the environment and the electronic system on the transitionfrom weak-to-strong coupling for different aromatic molecules. As part of thiswork, we also provide an easy-to-use tool to calculate the cavity couplingstrengths for simple cavity setups. Our work is a step towards parameter-freeab initio calculations for strongly coupled quantum light-matter systems andwill help bridge the gap between theoretical methods and experiments in thefield.