  The coherent interaction of a large collection of molecules with a common
photonic mode results in strong light-matter coupling, a feature that proved
highly beneficial for chemistry and termed the research topics polaritonic and
QED chemistry. Considering complex microscopic chemical reactions in
combination with a macroscopic number of molecules renders existing ab initio
approaches inapplicable. In this work, I introduce a simple approach to capture
the collective nature while retaining the full ab initio representation of
single molecules. By embedding the majority of the molecular ensemble into the
dyadic Green tensor, we obtain a computationally cheap and intuitive
description of the dynamic of a single molecule in the ensemble - an approach
that seems ideal for polaritonic chemistry. The introduced embedding
radiation-reaction potential is thoroughly discussed, including prospects,
applications and limitations. A first application demonstrates the linear
response of single molecules that are part of a larger ensembles of molecules.
Then, by virtue of a simple proton-tunneling model, I illustrate that the
influence of collective strong coupling on chemical reactions features a
nontrivial dependence on the number of emitters. Bridging classical
electrodynamics, quantum optical descriptions and the ab initio description of
realistic molecules, this work can serve as guiding light for future
developments and investigations in the quickly growing fields of QED chemistry
and QED material design.
