For a light-matter coupling strength of η c = 0.00125 a.u. , corresponding to a Rabi splitting Ω R ≈ 30cm -1 for the vibrational transitions |v mol 0 ± ⟩ → |v mol 1 ∓ ⟩, the fully quantummechanical results display a sharp resonant peak centered at ω c = 1185cm -1 . As elucidated in Ref. 51, in the singlemolecule limit, this resonant rate enhancement arises from the opening of an intramolecular reaction pathway, involving the molecular vibration, cavity mode, and its photon bath. The molecular vibrational transitions couple with the one-photon absorption/emission in the cavity mode, which is then quickly depleted/replenished by the photon bath to facilitate repeated occurrences of the former process. In comparison, the rates obtained with the mixed quantum-classical approach, where the cavity mode is treated classically, predict a slight reduction of the rates inside the cavity, devoid of any peak features.