However, the PN-QED-FCI-N p approach does not generally produce origin-invariant energies for charged species. We examine the energy error in the lowest-energy singlet state of H 2 O 2+ com-puted by PN-QED-FCI-N p while displacing the center of mass along the z-axis (see Figure , which is the polarization direction of the field and the direction along which dipole moment of H 2 O 2+ is oriented. We see that when N p = 1 (the minimal photon basis, including |0 p ⟩ and |1 p ⟩ photon number states), the energy approaches the milliHartree range by a displacement of 4 Angstroms from the origin and continues to increase with further displacement (see Figure ). We also clearly see the mitigating impact that growing the size of the photonic basis has on the origin dependence; for N p = 8, the error is in the microHartree range after a displacement of 20 Angstroms (see Figure ). We emphasize that since we are performing full CI within the electronic basis, these origin-dependent energy errors are arising solely from truncation of the photonic basis. As we approach the completeness of the photonic basis, we have a fully variational solution within the single electron orbital representation given by the 6-31G basis set, and see that the origin invariance of the energies is properly restored (see Figure ).