Two other peaks around 745cm -1 and 1645cm -1 start to emerge when the fourth vibrational level is included (N mol v = 5), and become much more apparent for N mol v = 6, as shown in Fig. ). The strongest peak centered at hot band 745cm -1 is related to the transition v mol 3 ↔ v mol 4 with ∆ε mol 3↔4 = 691cm -1 and v mol 2 ↔ v mol 5 with ∆ε mol 2↔5 = 806cm -1 . The peak in the high-frequency overtone regime can be assigned to the transition v mol 1 ↔ v mol 4 with ∆ε mol 1↔4 = 1593cm -1 and v mol 0 ↔ v mol 5 with ∆ε mol 0↔5 = 1704cm -1 , and it appears owing to the fast tunneling near the barrier top. Convergence is basically achieved when N mol v > 8, which means that higher vibrational levels barely contribute to the rate promotion.