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

We consider one of the most used model for water, the rigid four site TIP4P potential, and we study by molecular dynamics simulation the dynamical properties of the liquid upon supercooling. In the previous studies of the thermodynamics of the TIP4P model a liquid-liquid critical point (LLCP) located at the end of the coexistence between the low density liquid (LDL) and the high density liquid (HDL) of water was found. We present here the analysis of the self intermediate scattering functions in a large range of temperatures and densities and we show that the structural relaxation in the region of mild supercooling is in agreement with the predictions of the mode coupling theory. In the more deep supercooled region we observe that the α-relaxation time deviates from the mode coupling theory (MCT) trend and a crossover takes place from a fragile to a strong behavior upon crossing the Widom line emanating from the LLCP. The HDL and the LDL phases are associated with the fragile and the strong behavior, respectively.