Next, we investigate the case with the larger mass, M = 2 a.u. , while keeping all other parameters fixed. In this scenario (Model II), the energy gaps between vibrational levels become narrower and the zero-point energy (the energy of the lowest two degenerate eigenstates) is shifted further away from the barrier (see Table ). As a consequence, there are six states below the energy barrier, as shown in Fig. . The tunneling rate for N mol v = 2 is significantly reduced to 1.5 × 10 -9 fs -1 . The states |v mol 2 ⟩ and |v mol 3 ⟩ are nearly degenerate. However, while the inclusion of state |v mol 2 ⟩ only slightly increases the reaction rate, the involvement of both, in great contrast, leads to a significant speedup by two orders of magnitude. The rate for N mol v = 4 is 1.9 × 10 -7 fs -1 . Unlike Model I, we observe in Fig. ) another notable leap in the reaction rate when the states |v mol 4 ⟩ and |v mol 5 ⟩ are also included. This is due to two extra cotunneling paths, 4 and 5 , as shown in Fig. . Path 4 comprises a pair of solvent-induced vibrational excitation and relaxation between the states |v mol 3 ⟩ ↔ |v mol 4 ⟩, which is allowed by symmetry (see Fig. )), as well as the shallow tunneling at the near-barrier vibrational state |v mol 4 ⟩. Similarly, Path 5 is a cotunneling process that involves the vibrational transition |v mol 2 ⟩ ↔ |v mol 5 ⟩ and the tunneling at |v mol 5 ⟩. The reaction rate converges when at least the lowest eight eigenstates are considered, and the interference between all possible reaction paths results in a rate k = 3.3 × 10 -7 fs -1 , which is six times smaller than that of Model I.