Note that this is the quantum description of the reaction based on quantized states, whereas the classical description is a barrier crossing along the reaction coordinate. The mechanism for the reaction is that the thermal activation process causes the transition of |ν L ⟩ → |ν ′ L ⟩. Then the reaction occurs through the diabatic couplings between |ν ′ L ⟩ and |ν ′ R ⟩, followed by a vibrational relaxation of the product state, |ν ′ R ⟩ → |ν R ⟩. The rate-limiting step for the entire process is k 1 , where k 2 ≫ k 1 such that the populations of both |ν ′ L ⟩ and |ν ′ R ⟩ reach a steady state (plateau in time), and from the steadystate approximation, the overall rate constant for the reaction is k 0 ≈ k 1 (see Appendix B in Ref. 9 for detailed discussion).