Static and Dynamic Properties of Stretched Water

We present the results of molecular dynamics simulations of the extended simple point charge (SPC/E) model of water to investigate thethermodynamic and dynamic properties of stretched and supercooled water. We locate the liquid-gas spinodal, and confirm that the spinodal pressure increases monotonically with $T$, supporting thermodynamic scenarios for the phase behavior of supercooled water involving a ``non-reentrant'' spinodal. The dynamics at negative pressure show a minimum in the diffusion constant $D$ when the density is decreased at constant temperature, complementary to the known maximum of $D$ at higher pressures. We locate the loci of minima of $D$ relative to the spinodal, showing that the locus is inside the thermodynamicaly metastable regions of the phase-diagram. These dynamical results reflect the initial enhancement and subsequent breakdown of the tetrahedral structure and of the hydrogen bond network as the density decreases.