C 8 H 10 Our final system of study is the naphthalene molecule (C 8 H 10 ) coupled to a photon tuned to the transition between the ground-state and the second singlet excited state ( 0 S and 2 S). This represents the simplest embodiment of the polycyclic aromatic hydrocarbons (PAH) family of molecules that are paradigmatic molecular systems for strong multireference correlation effects. Cavity effects in aggregates of PAH systems have also been proposed as a route to enhance singlet fission and inversion of singlet-triplet gaps. Here we investigate the ordering of singlet polariton states relative to a nearby triplet state using our CS-QED-CASCI approach. Although we consider only a single molecule coupled to a cavity, we can envision that such calculations may enable the parameterization of accurate models for aggregates to enable future studies. We optimize the geometry at the MP2/cc-pVTZ level of theory using density fitting, and at this geometry, we find the second singlet excited state at both the CASCI(10,10)/cc-pVDZ and CASCI (12,12)/cc-pVDZ level has a transition energy of 5.92 eV and a strong transition dipole moment along the y-axis. The (10,10) active space includes all electrons and orbitals comprising the aromatic system; however, we find that the evolution of the polariton energies becomes non-smooth for values of λ y > 0.01 atomic units in this active space, while we observe smooth evolution of the energies for all values of λ y considered when a (12,12) active space is used.