Excitonic polarons and self-trapped excitons from first principles calculations, without supercells*

Excitons are neutral excitations that are composed of electrons and holes bound together by their attractive Coulomb interaction. Excitons also interact with the underlying atomic lattice, which leads to a trapping potential that favors exciton localization. The quasi-particle thus formed by the exciton and the surrounding lattice distortion is called excitonic polaron. Excitonic polarons have long been thought to exist in a variety of materials, but quantitative ab initio calculations of these effects are exceedingly rare.  Here, we present a theory of excitonic polarons that is amenable to first-principles calculations. We apply this theory to calculating excitonic polarons in LiF and Cs2ZrBr6 ab initio. The key advantage of the present approach is that it does not require supercells, therefore it can be used to study a variety of materials hosting either small or large excitonic polarons. This work constitutes the first step toward a complete ab initio many-body theory of excitonic polarons in real materials.