Diffusion dynamics of supercooled water modeled with the cage-jump motion and hydrogen-bond rearrangement

The slow dynamics of glass-forming liquids is generally ascribed to the cage jump motion. In the cage jump picture, a molecule remains in a cage formed by neighboring molecules and, after a sufficiently long time, it jumps to escape from the original position by cage breaking. The clarification of the cage jump motion is therefore linked to unraveling the fundamental element of the slow dynamics. Here, we develop a cage jump model for the dynamics of supercooled water. The caged and jumping states of a water molecule are introduced with respect to the hydrogen-bond (H-bond) rearrangement process and describe the motion in supercooled states. It is then demonstrated from the molecular dynamics simulation of the TIP4P/2005 model that the characteristic length and time scales of cage jump motions provide a good description of the self-diffusion constant that is determined in turn from the long-time behavior of the mean square displacement. Our cage jump model thus enables the connection between H-bond dynamics and molecular diffusivity.