We consider a molecule with two electronic states ĤM = T + E g (R)|g g| + E e (R)|e e|, (45) 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 oppose to the atomic case where they are always zero. Hence, it is not always a good approximation to drop them. The Rabi model assumes that one can drop the permanent dipoles, and leads to the dipole operator expression in the subspace P = |g g| + |e e| as follows P μ P = µ eg (|e g| + |g e|) ≡ µ eg (σ † + σ), (46) where we have defined the creation operator σ † ≡ |e g| and annihilation operator σ ≡ |g e| of the electronic excitation. The PF Hamiltonian (Eq. 44) in the subspace P thus becomes ĤPF = ĤM + Ĥph +χ•µ eg (σ † +σ)(â † +â)+ 1 hω c (χ•µ eg ) 2 .