where ĤS is now diagonal. Herrera and Spano had shown that strong collective resonant coupling of a cavity field with N exciton transitions can effectively decouple exciton-phonon couplings in a disordered molecular ensemble. This has also been observed experimentally . To be specific, the coupling strength is re-scaled as c α / √ N for both the diagonal (Holstein coupling) and off-diagonal (Peierls coupling) terms associated with the polariton states |+⟩ and |-⟩ as shown in Eq. 26c. In particular, under the resonance condition, the mixing angle is Θ N = π/4. As a result, the displacement between the |G, 0⟩ and the |±⟩ states is given by