such that 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 breakdown of the quantum optics models for computing polariton potential energy surface will be discussed in Sec. 3.1.3 The Rabi model assumes that one can ignore the permanent dipole moments (PD), and leads to the dipole operator expression in the subspace P = |g⟩⟨g| + |e⟩⟨e| as follows