Predictions of dynamic behavior under pressure for two scenarios to explain water anomalies.

Using Monte Carlo simulations and mean field calculations for a cell model of water we find a dynamic crossover in the orientational correlation time tau from non-Arrhenius behavior at high temperatures to Arrhenius behavior at low temperatures. This dynamic crossover is independent of whether water at very low temperature is characterized by a "liquid-liquid critical point" or by the "singularity-free" scenario. We relate tau to fluctuations of hydrogen bond network and show that the crossover found for tau for both scenarios is a consequence of the sharp change in the average number of hydrogen bonds at the temperature of the specific heat maximum. We find that the effect of pressure on the dynamics is strikingly different in the two scenarios, offering means to distinguish between them.