The left panel of Figure shows the CS-QED-FCI-10/6-311G polariton surfaces (lower-polariton (LP) in red, upper-polariton (UP) in blue) along with the cavity-free FCI/6-311G surfaces for the As a final analysis on the LiH system, we investigate the behavior of the ground-state potential energy surface as a function of the active space and photonic basis size for the QED-CASCI method. Specifically, we compute the ground-state potential energy surfaces for the LiH cavity system at the CS-QED-CASCI(4,n)-N p /6-311G and PN-QED-CASCI(4,n)-N p /6-311G levels with n = [3, 15] and N p = 1 and 10; note that a (4,16) active space is identical to QED-FCI for LiH in a consistently lower than the PN-QED-CASCI(4,n)-10 results for all except the (4,5) active space as well. This suggests that there can be additional sensitivity to the size of the photon basis when the electronic excitation space is truncated that was not evident in the QED-FCI results presented in Figure . with hω = 10 eV with a polarization vector λ = (0, 0, 0.01) in atomic units. There are no opticallyallowed transitions in the UV-Vis region for this system; the photon energy hω is not resonant with any transitions from the FCI/6-31G eigenspectra in this species and is arbitrarily chosen. We keep the geometry fixed at r OH = 1.0 Angstroms and θ HOH = 104.5 • , which is a typical geometry for neutral water. While this will surely not correspond to an equilibrium geometry for H 2 O 2+ , we simply seek to study the behavior of the QED-CASCI method for a charged closed-shell system for which we can provide a QED-FCI benchmark. Charged molecular species, unlike their neutral counterparts, have origin-dependent dipole moments, which leads to origin-dependent energy errors in certain formulations of ab initio QED methods, including QED-HF and QED-CC in the photon number basis. The origin-dependence of QED-HF can be completely alleviated through saturation of the photon basis set or, alternatively, through formulating QED-HF in the coherent-state basis. Similarly, we observe that origin-invariant energies are obtained from the CS-QED-FCI-1 approach.