  We perform molecular dynamics simulations using the extended simple point
charge SPC/E water model in order to investigate the dynamical behavior of
supercooled-stretched water. We focus on the behavior of the translational
diffusion coefficient, the orientational relaxation time, and the local
hydrogen bond network. Decreasing density or pressure along an isothermal path,
there is a mininum in the diffusion coefficient and a maximum in the
orientational relaxation time, suggesting an initial enhancement and subsequent
breakdown of the tetrahedral structure and of the hydrogen bond network as the
density decreases. The analysis of the tetrahedral structure of the nearest
neighbors help to clarify the relationship between the local structural changes
and the system dynamics. We also find that the product of diffusion coefficient
and relaxation time is nearly constant, indicating that the anomalous behavior
observed in the rotational and translational diffusion arise from the same
microscopic mechanism.
