Single particle dynamics of supercooled water and the kinetic glass transition

Abstract

A molecular dynamics simulation in a time interval of fs to 50 ns has been performed to study the single-particle dynamics of supercooled SPC/E water. In spite of the fact that, upon supercooling, SPC/E water progressively evolves into a more open structure locally, the single particle dynamics is shown to be dominated by the so called cage effect. The structural relaxation of the cage at low temperatures leads to the phenomenon of slow dynamics that can only be completely studied by following the trajectories into the ns range. Such results can be interpreted in the framework of the mode-coupling theory for supercooled liquids. The so called critical Angell temperature of supercooled water can thus be interpreted as kinetic glass transition temperature. For this simulation various approximation previously used in literature for the evaluation of the intermediate scattering function from the experimental results are no longer valid. In fact upon supercooling the stochastic single-particle diffusion looses its validity and the diffusion process is then progressively controlled by the structural relaxation of the cage. The possibility of an experimental test of such a behaviour on real supercooled water is explored.