where r 1 (C -C) and r 2 (C -C) the distances corresponding to the two C-C bonds broken in the reaction (see the inset (a) of Figure ). This definition gave the µ q of 3.0 a.m.u. Nine MTD trajectories with the bias potential update time of 10 fs and width σ = 0.1 Å, resulting in bias energies E b = 281 cm -1 , with total simulation duration of 5 ps. To prevent the dissociation of the ethylene and 1,3-butadiene, an artificial constraining potential was applied. It was defined as