IS THERE A 2ND CRITICAL-POINT IN LIQUID WATER

The supercooled and stretched regions of the phase diagram of simulate d liquid water are investigated by calculating the equation of state o f the ST2 and TIP4P pair-potentials. We find that simulated water does not display a re-entrant spinodal and that the projection of the dens ity maximum line in the plane of pressure and temperature becomes posi tively sloped on stretching. The well-known anomalous behavior of supe rcooled water is tentatively associated with the existence of an inacc essible critical point. Evidence is presented that suggests the associ ation of this new critical point with the transition between low densi ty and high density amorphous solid water. We show how the observed tr ansformation behavior of the two forms of amorphous solid water can be explained in terms of a first order phase transition, via a considera tion of the limits of metastability associated with this kind of trans ition, and support this interpretation with simulations of the amorpho us solid. We therefore propose a phase diagram which accounts for the behavior of both liquid and amorphous solid water.