Rate Constant Calculations. We use hierarchical equations of motion (HEOM) to simulate the population dynamics and obtain the VSC-modified rate constant, see details in Supplementary Note 1-3. Here, we treat ĤM as the quantum subsystem and represent it using the vibrational eigenstates {|ν 0 ⟩, |ν 1 ⟩, ...}, and the rest terms in the Hamiltonian are treated as the bath in HEOM, see details in Supplementary Note 2. The population dynamics of the "reactant" is computed as P R (t) = Tr S [(1 -ĥ)ρ S (t)], where the trace Tr S is performed along the system DOF, and ĥ = h( R0 -R ‡ ) is the Heaviside operator with R ‡ = 0 as the dividing surface for the model potential V ( R) (in Eq. 25). The forward rate constant is obtained by evaluating k = -lim t→tp ṖR (t)