in terms of the deviation of the molecular dipole from its expectation value in the ground state ∆ μ = μ -⟨ μ⟩ GS and its variance (∆ μ) . In the following section (Sec. 3.2.2), we will see that ⟨∆ μ⟩ SD = 0 for many-body methods that employ only a single-determinant (SD) wavefunction, such as Hartree-Fock (SD→HF) or density functional (SD→DFT) theories, allowing for a drastic simplification of the coupled electron-photon Hamiltonian. Note that contributions from the dipole self-energy survive, since ⟨ (∆ μ) 2 ⟩ SD = ⟨ μ2 ⟩ SD -⟨ μ⟩ 2 SD , even with a single determinant approach.