Solid-fluid equilibrium for a molecular model with short ranged directional forces

The phase diagram of a system of hard spheres with short-range tetrahedral association has been determined by computer simulation and theory. The fluid phase and two solid phases were considered. One of these solid phases is a low-density solid closely related in structure to ice Ic and the other is a high-density solid closely related in structure to ice VII. At high temperatures freezing occurs into the high-density solid whereas at low temperatures freezing occurs into the low-density solid. At an intermediate temperature a triple point is found where the fluid coexists with the two solids simultaneously; Although the low-density solid melts to a high-density fluid, this transition is found to be metastable with respect to the transformation into a high-density solid. This is evidence that short-range tetrahedral attractive forces are not in and of themselves sufficient to explain the anomalous melting of water. Our results indicate that vapor-liquid equilibrium for the model is preempted by solidification. Monte Carlo simulation results for the fluid phase are described successfully by Wertheim's theory whereas those of the solid phases are described qualitatively by the cell, theory. (C) 1998 American Institute of Physics. [S0021-9606(98)52346-X].