High-temperature dynamic behavior in bulk liquid water： A molecular dynamics simulation study using the OPC and TIP4P-Ew potentials

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＊ ≈ 309 and T＊ ≈ 285 K for the OPC and TIP4P-Ew models, respectively. Experimen- tally, the crossover point was previously observed at T＊ ≈ 315 ±5 K. We also verified that for the coefficient of thermal expansion αp（T, P）, the isobaric αp（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.