Ref. 70 also investigated the cavity modification of a non-adiabatic electron transfer reaction when photon frequency is very low, such that the classical description of the photon mode becomes accurate. With a classical treatment of the photon mode, the diabatic coupling becomes time-dependent and is a function of the photon coordinate, such that V DA (q c ) = V 0 DA + 2ω 3 c A 0 µ DA q c , In such a case, the photon mode plays the role of a Peierls coupling mode. For such fluctuating diabatic coupling the chemical rate is given by