We consider several model closed-shell systems for which we can establish an exact benchmark using QED-FCI with a saturated photon basis, including LiH in a 6-311G basis set, H 2 O 2+ cation and BH 3 in a 6-31G basis set. This selection of model systems includes a non-polar (but polarizable) molecule (BH 3 ), a polar molecule (LiH), and a molecule with net charge (H 2 O 2+ ), which engenders a range of coupling behavior between the molecule and the photon field. After analyzing the behavior of both formulations of QED-CASCI for these three systems with respect to an exact benchmark, we conclude with an illustrative calculation of the naphthalene molecule treated in the cc-pVDZ basis set. Here, we explore the behavior of singlet polariton states for a range of coupling strengths leading to inversion of a singlet-triplet ordering in a (12,12) active space, and also illustrate the scaling of our approach with respect to the number of photon states within this active space.