When dealing with the full molecular cavity QED situation, where both the permanent and transition dipoles (Eq. 28) need to be considered, the coupling strength g c or η (Eq. 84) no longer accurately describes the systems because it only includes a particular value of the transition dipole, whereas both transition and permanent dipoles could change their values significantly as a function of the nuclear coordinate in a real molecular system (see example in Fig. ). For this case, typically two different expressions for coupling parameters are used in the literature, either the magnitude of the vector potential, A 0 = ℏ 2ωcϵ0V , or a coupling parameter that does not explicitly depend on the cavity frequency