SPINODAL OF LIQUID WATER

An open question in the study of water concerns the shape of the liquid spinodal line in the phase diagram of water, a boundary which represents the limit of mechanical stability of the liquid state. It has been conjectured that the pressure of the liquid spinodal P(s)(T) does not decrease monotonically with decreasing temperature T, but passes through a minimum and is ''reentrant'' from negative to positive pressure P in a region of T in which the liquid is deeply supercooled. The conjectured minimum in P(s)(T) has not been directly observed due to the difficulties encountered in experiments which attempt to study liquid water under tension. Here we exploit the ability of molecular-dynamics computer simulations to model the behavior of liquid water deep into its metastable region. We thereby attempt to observe a minimum in P(s)(T). We first argue that the ST2 potential of Stillinger and Rahman [J. Chem. Phys. 60, 1545 (1974)] is the best of several commonly used water interaction potentials for this purpose. Then, we conduct simulations of a system of ST2 particles over a wide range of stable and metastable liquid-state points, and demonstrate that P(s)(T) for ST2 is not reentrant. In a second set of simulations we test if the behavior we find is limited to the ST2 potential by exploring the relevant thermodynamic region of the liquid as simulated by the TIP4P interaction potential of Jorgensen et al. [J. Chem. Phys. 79, 926 (1983)]. We find that the TIP4P potential confirms the absence of a reentrant spinodal. We also show how the structural and energetic properties of both the ST2 and TIP4P liquids are consistent with the absence of a reentrant spinodal.