  In the last decade, much theoretical research has focused on studying the
strong coupling between organic molecules (or quantum emitters, in general) and
light modes. The description and prediction of polaritonic phenomena emerging
in this light-matter interaction regime have proven to be difficult tasks. The
challenge originates from the enormous number of degrees of freedom that need
to be taken into account, both in the organic molecules and in their photonic
environment. On the one hand, the accurate treatment of the vibrational
spectrum of the former is key, and simplified quantum models are not valid in
many cases. On the other hand, most photonic setups have complex geometric and
material characteristics, with the result that photon fields corresponding to
more than just a single electromagnetic mode contribute to the light-matter
interaction in these platforms. Moreover, loss and dissipation, in the form of
absorption or radiation, must also be included in the theoretical description
of polaritons. Here, we review and offer our own perspective on some of the
work recently done in the modelling of interacting molecular and optical states
with increasing complexity.
