and the transition dipole is µ eg = ⟨e| μ|g⟩. Note that the permanent dipoles in a molecule µ ee = ⟨e| μ|e⟩, µ gg = ⟨g| μ|g⟩ are not necessarily zero, as opposed to the atomic case where they are always zero. Hence, it is not always a good approximation to drop them. The Rabi model, however, assumes that one can ignore the permanent dipole moments (PDM) and leads to the dipole operator expression in the subspace P = |g⟩⟨g| + |e⟩⟨e| of