FGR rate theory in the lossless regime. When the cavity is under the lossless regime (τ c ≫ Ω -1 R ), the rate-limiting step of the reaction becomes the photonic excitation |0⟩ → |1⟩ and the subsequent excitation energy transfer (see Fig. ). The VSC enhancement thus originates from the enhancement of the photonic excitation caused by the photon-loss bath Ĥc , as proposed in Ref. . Under this regime, the numerically exact HEOM simulations suggest the following reaction mechanism (schematically depicted in Fig. )