Water at Positive and Negative Pressures

TL;DR

This paper reviews molecular dynamics simulations of water models, revealing complex phase behavior including a non-monotonic TMD line, a low-temperature phase transition, and unique diffusion dynamics at negative pressures.

Contribution

It provides new insights into water's phase diagram and dynamics at negative pressures using advanced simulation models, especially TIP5P.

Findings

Existence of a non-monotonic TMD line with a crossover at ambient pressure.

Presence of a low-temperature phase transition in water.

Diffusion constant D shows a minimum at negative pressures.

Abstract

We review recent results of molecular dynamics simulations of two models of liquid water, the extended simple point charge (SPC/E) and the Mahoney-Jorgensen transferable intermolecular potential with five points (TIP5P), which is closer to real water than previously-proposed classical pairwise additive potentials. Simulations of the TIP5P model for a wide range of deeply supercooled states, including both positive and negative pressures, reveal (i) the existence of a non-monotonic ``nose-shaped'' temperature of maximum density (TMD) line and a non-reentrant spinodal, (ii) the presence of a low temperature phase transition. The TMD that changes slope from negative to positive as P decreases and, notably, the point of crossover between the two behaviors is located at ambient pressure (temperature approx 4 C, and density approx 1 g/cm^3). Simulations on the dynamics of the SPC/E model reveal (iii) the dynamics at negative pressure shows 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, and (iv) the loci of minima of D relative to the spinodal shows that they are inside the thermodynamically 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.