Molecular-dynamics study of translational motions in water as probed through quasielastic neutron scattering

Results from computer molecular-dynamics simulations of pure water over a temperature range from 80 °C down to -30 °C are reported. The principal emphasis is on the nature of the translational diffusive motion of water molecules as deduced from quasielastic neutron scattering (QENS) experiments. The behavior of the QENS translational linewidth as a function of momentum transfer, q, is determined up to q=5.0–6.0 A${̊}^{\mathrm{-}1}$ and compared with experimentally derived linewidths up to q=2.0 A${̊}^{\mathrm{-}1}$ at several temperatures. Conclusions drawn from these comparisons reinforce those from earlier molecular-dynamics studies which found that self-diffusion in water takes place in a fairly continuous fashion in contrast to that implied by a random jump diffusion model. This is true even though that model does give a good description of recent high-quality QENS data.