where r (O -H) and r (O • • • H) denote the distances between oxygens and hydrogen, corresponding to the O-H covalent and O• • • H hydrogen bonds in the equilibrium geometry, respectively (see Fig. ). The reduced mass of this CV was thus µ q = 1.9 a.m.u. The width of the bias potential σ was 0.2 Å, which subsequently gave the bias energy E b (Eq. 5) of 111 cm -1 . The bias potential was updated with the current value of CV every 30 fs. We accumulated 16 MTD trajectories in total, each 2 ps in duration.