At each temperature of 300, 500, 1000 K, 2~7 x10 5 trajectories were carried out to make sure that the corresponding statistical errors are all less than 5%. All trajectories were launched with the initial separation between two reactants being 10 Å and halted once the distance between two fragments reaches 10.5 Å. The maximum of impact parameter bmax was determined by increasing the impact parameter b gradually until no reaction takes place in a small batch of trajectories (often 10 4 ). Then for each trajectory, the impact parameter b was chosen from a range from 0 to bmax, according to the formula