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 are compared with high-resolution quasi-elastic neutron scattering (QENS) data. The simulations are performed at 264 K using the extended simple point charge model. The MD results indicate that after less than 0.1 ps the proton dynamics practically coincides with the centre of mass dynamics, showing thus a substantial coupling between translational and reorientational motions. Simple diffusion models are found inadequate to describe the MD and QENS results. The centre of mass self-dynamic structure factor has been analyzed in terms of a second-order memory function model. The model identifies two characteristic relaxation times; 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.