High-Temperature Dynamic Behavior in Bulk Liquid Water: A Molecular Dynamics Simulation Study using the OPC and...

Classical molecular dynamics simulations were performed to study the high-temperature (above 300 K) 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 $T^{*}\approx 309$ and $T^{*}\approx 285\text{K}$ for the OPC and TIP4P-Ew models, respectively. Experimentally, the crossover point was previously observed at $T^{*}\approx 315\pm 5\text{K}$ . We also verified that for the coefficient of thermal expansion ${\alpha }_P(T,P)$ , the isobaric ${\alpha }_P\left(T\right)$ 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.