First, we consider the effective molecular mass as M = 1 a.u. , which is also used in the prior studies. In this case (Model I), there are four vibrational eigenstates below the barrier, as illustrated in Fig. . The transition between different vibrational eigenstates is caused by the solvent, represented by a bilinear system-bath interaction in Eq. (10). Thus, the transition probability between the states |v mol k ⟩ and