MOLECULAR-DYNAMICS STUDY OF SINGLE-PARTICLE MOTION IN SUPERCOOLED WATER - MEMORY FUNCTION-APPROACH AND COMPARISON WITH QUASI-ELASTIC NEUTRON-SCATTERING

The single-particle dynamics of liquid water in supercooled regime has been studied by molecular dynamics (MD) simulations and the results a re compared with high-resolution quasi-elastic neutron scattering (QEN S) data. The simulations are performed at 264 K using the extended sim ple point charge model. The MD results indicate that after less than 0 .1 ps the proton dynamics practically coincides with the centre of mas s dynamics, showing thus a substantial coupling between translational and reorientational motions. Simple diffusion models are found inadequ ate to describe the MD and QENS results. The centre of mass self-dynam ic structure factor has been analyzed in terms of a second-order memor y function model. The model identifies two characteristic relaxation t imes; the short one is probably related to the hindered motion of the particle within the nearest-neighbour ''cage'', while the long one can be assigned to the dynamics of the relaxation of water clusters.