A reactive force field molecular dynamics simulation of the dynamic properties of hydrogen bonding in supercritical water

Dynamic properties especially lifetime and connectivity of hydrogen bonds in supercritical water play essential roles in physical and chemical properties in aqueous systems. Although average lifetime of hydrogen bonds in supercritical water derived using computer simulation has been reported, detailed distribution and survival analyses have yet to be performed. Here, we performed classical molecular dynamics (MD) simulations with a recently developed reactive force field method to investigate the detailed dynamic process of hydrogen bond forming, cracking, re-bonding and exchanging in supercritical water. We analyzed the MD-calculated trajectories with a sample interval equal to the MD time-step to obtain detailed lifetime information. The calculated lifetime distributions indicate that hydrogen bonds in supercritical water exhibit the newborn instability which is independent of temperature. Supercritical water shows non-exponential kinetics with a gradually decreasing failure rate. We also investigated the connectivity of hydrogen bonding by analyzing the fractions of water molecules involved in a certain number of hydrogen bonds and found that the fractions at high densities remarkably deviate from the widely applied simple binomial distribution. On the basis of the assumption that the maximal coordination number can be a non-integer w, we proposed a non-integer binomial distribution with a maximum coordination number of 4.4 for an enhanced prediction.