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Among the numerous anomalies of water, the acceleration of dynamics underpressure is particularly puzzling. Whereas the diffusivity anomaly observed inexperiments has been reproduced in several computer studies, the parallelviscosity anomaly has received less attention. Here we simulate viscosity andself-diffusion coefficient of the TIP4P/2005 water model over a broadtemperature and pressure range. We reproduce the experimental behavior, andfind additional anomalies at negative pressure. The anomalous effect ofpressure on dynamic properties becomes more pronounced upon cooling, reachingtwo orders of magnitude for viscosity at 220 K. We analyze our results with adynamic extension of a thermodynamic two-state model, an approach which hasproved successful in describing experimental data. Water is regarded as amixture of interconverting species with contrasting dynamic behaviors, onebeing strong (Arrhenius), and the other fragile (non-Arrhenius). The dynamicparameters of the two-state models are remarkably close between experiment andsimulations. The larger pressure range accessible to simulations suggests amodification of the dynamic two-state model, which in turn also improves theagreement with experimental data. Furthermore, our simulations demonstrate thedecoupling between viscosity $\eta$ and self-diffusion coefficient $D$ as afunction of temperature $T$. The Stokes-Einstein relation, which predicts aconstant $D \eta/T$, is violated when $T$ is lowered, in connection with theWidom line defined by an equal fraction of the two interconverting species.These results provide a unifying picture of thermodynamics and dynamics inwater, and call for experiments at negative pressure.

Viscosity and self-diffusion of supercooled and stretched water from molecular dynamics simulations