To elucidate the impact of the cavity photon mode on reaction dynamics, we place a single molecule inside an optical cavity and vary the cavity photon frequency. Additionally, we assume that the molecular dipole moment is a linear function of the reaction coordinate and aligned in the same direction as the light polarization, i.e. ⃗ µ(x) ⋅ ⃗ e = x, where the coefficient is absorbed into η c . The Rabi splitting corresponding to a vibrational transition between the states |v mol i ⟩ and |v mol j ⟩ in the resonant condition can be roughly estimated as Ω R = 2ω c η c |µ i j |, where µ i j = ⟨v mol i |x|v mol j ⟩. A more detailed analysis of the energy level of several polaritonic states and Rabi splittings is provided in the SI.