Simulation Study on the Diffusive Motion in Deeply Supercooled Water

The diffusion process in supercooled liquid water has been studied by a series of molecular dynamics simulations using systems of 216 ST2-model molecules in the temperature range between 255 and 360 K. In contrast to SPC/E water ( Phys. Rev. E 1996, 54, 6331), the ST2 liquid does not show the appearance of a kinetic glass transition with structural arrest. Instead, we observe a significant change of the diffusion mechanism in the deeply supercooled region. The high-temperature microstep diffusion mechanism transforms continuously into a jump-diffusive behavior at low temperatures. By analyzing the intermediate incoherent structure factor of the center of mass motion we can characterize the hopping process by an average residence time τ 0 . The hopping process is found to control reorientational and translational motions, leading to a second region of apparent Arrhenius behavior with a high activation energy of about 115 kJ mol -1 . The change in the dynamic behavior occurs in parallel to the structural and thermodynamical transformation to a low-density liquid form of ST2 water.