where V ( R) is expressed in Eq. 20. These vibrational eigenstates are obtained using the discrete variable representation (DVR) basis with 1001 grid points in the range of -2.0 ≤ R ≤ 2.0. In the quantum dynamics simulations using HEOM, we treat the number of vibrational eigenstates as a convergence parameter, and we have included a total of 10 states (from |ν 0 to |ν 9 ). Details (including the numerical convergence testing results) are provided in Sec. III of the Supplementary Material.