[2]. The current theory assumes only one cavity mode, whereas in the FabryPérot cavity, there are many cavity modes. For a k ⊥ mode that satisfies mirror boundaries, there will be continuous choices of k , such that the photon energy is ω k ∝ k 2 + k 2 ⊥ . Experimentally, only when ω k = ω 0 at k = 0 is satisfied can one observe VSC modification on the rate constant. For a given finite k , it is possible for ω k = ω 0 , but there will be no apparent VSC effect. 6,7,79 [3]. The current work focuses on the VSC enhancement where the reaction is originally in the energy diffusion-limited regime (low friction regime before the Kramers turnover ). Although we have not discussed VSC-suppressed reactivity, the cavitysuppressed steady-state population and the rate constant k 1 have been observed in classical and quantum simulations , when the molecular system is originally under the high friction limit (after Kramers turnover , or the so-called spatial diffusion-limited regime). We anticipate that the current theory and simulations will also likely be able to produce the resonance suppression effect if the reaction is originally sitting in the spatial diffusionlimited regime.