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 steady-state approximation, the overall rate constant for the reaction is 𝑘 0 ≈ 𝑘 1 . This steady-state behavior of the |𝜈 ′ L ⟩ and |𝜈 ′ R ⟩ states has recently been verified by numerically exact quantum dynamics simulations .