Mode coupling theory and fragile to strong transition in supercooled TIP4P/2005 water.

We study by molecular dynamics simulations supercooled water with the TIP4P/2005 potential. This model is able to predict many properties of water in a large range of the thermodynamic space in agreement with experiments. We explore the dynamical behavior and, in particular, the self intermediate scattering function of the oxygen atoms. We find that the structural relaxation in the range of mild supercooling is in agreement with the Mode Coupling Theory (MCT). The ideal MCT crossover takes place at decreasing temperature with increasing density. Deviations from the MCT behavior are found upon further supercooling. A crossover from the MCT, fragile, regime to a strong, Arrhenius, regime is found and it is connected to the presence of a liquid-liquid phase transition and the Widom line emanating from the liquid-liquid critical point.