High-Temperature Dynamic Behavior in Bulk Liquid Water: a Molecular Dynamics Simulation Study Using OCP and TIP4P-Ew Potentials

Classical molecular dynamics simulations were performed to study the high-temperature (above 300K) dynamic behavior of bulk water, specifically the behavior of the diffusion coefficient,hydrogen bond, and nearest-neighbor lifetimes. Two water potentials were compared: the recently proposed “globally optimal” point charge (OPC) model and the well-known TIP4P-Ew model. By considering the Arrhenius plots of the computed inverse diffusion coefficient and rotational relaxation constants, a crossover from Vogel–Fulcher–Tammann behavior to a linear trend with increasing temperature was detected at T c.a. 309 and T c.a. 285 K for the OPC and TIP4P-Ew models, respectively. Experimentally, the crossover point was previously observed at Tc.a. 315 +- 5 K. We also verified that for the coefficient of thermal expansion AlphaP (T; P), the isobaric AlphaP (T) curves cross at about the same T* as in the experiment.
The lifetimes of water hydrogen bonds and of the nearest neighbors were evaluated and
were found to cross near T*, where the lifetimes are about 1 ps. For T < T*, hydrogen bonds persist longer than nearest neighbors, suggesting that the hydrogen bonding network dominates the water structure at T < T*, whereas for T > T*, water behaves more like a simple liquid. The fact that T* falls within the biologically relevant temperature range is a strong motivation for further analysis of the phenomenon and its possible consequences for biomolecular systems.