Here, we have introduced C = 2ω 3 c ℏ A 0 µ ′ ‡ which characterizes the light-matter coupling strength. Clearly, the coupling strength between the reactive molecule and the cavity mode is limited by C (single-molecule coupling strength). As a result, the reaction rate will not depend on the number of molecules. The same conclusion is drawn when using the Pollak-Grabert-Hänggi theory that extends the MTST to the energy diffusion-limited regime. Then the normal mode frequencies are used to compute κ N , which is the ratio between the rate constant of N molecules inside the cavity and the TST rate of one molecule outside the cavity,