Effect of the Rabi splitting. We further explore the effect of the light-matter coupling strength on the VSC rate constant and the accuracy of the FGR expression in Eq. 19 (Eq. 17 under the resonance condition) in the lossless regime. By doing so, we fix the cavity lifetime as τ c = 1000 fs. Fig. presents the relation between k/k 0 at resonance (ω c = ω 0 ) under various Rabi splitting Ω R , obtained from the HEOM simulations (red circles) and the FGR expression (red solid line) using kVSC in Eq. 19 with a re-scaling factor of 0.5 on kVSC . Over up to 100 cm -1 Rabi splitting, the FGR expression (Eq. correctly captures the Ω R -dependence that first scales as kVSC ∝ Ω 2 R , then plateau (saturated). This is because when Ω R becomes large, only k1 (Eq. 12) is rate limiting, which is Ω R -independent. Fig. further presents the change of the effective free energy barrier ∆(∆G ‡ ), directly calculated from the rate constant ratio k/k 0 obtained from HEOM simulations. To account for the "effective change" of the Gibbs free energy barrier ∆(∆G ‡ ) as follows