Starting points for uphill trajectory were obtained by independent ACE-MD simulations of reactants at 300 K with r 1 and r 2 constrained by harmonic potentials at 3.0 ∼ 5.0 Å, respectively. The uphill trajectories were propagated with a harmonic potential (spring constant of 0.4 eV Å -2 ) to overcome the energy barriers. Trajectories were propagated until products formed or the simulation time reached 3 ps.