where C j αn = ⟨ψ α (R), n|Φ j (R)⟩. Here, the number of included electronic states, N el , and photonic Fock/number states, N F , are treated as convergence parameters. A convergence test is provided in the Supporting Information, and the typical numbers of states are N F = 5 and N el = 50. Further details regarding the pQED approach are provided in the Supporting Information. The accuracy of the above-described pQED approach has been benchmarked with the more accurate selfconsistent QED coupled-cluster (scQED-CC) approach, where the pQED method generates nearly quantitative agreement with the scQED-CC approach.