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 flui d 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 temp eratures freezing occurs into the high-density solid whereas at low tempera tures freezing occurs into the low-density solid. At an intermediate temper ature a triple point is found where the fluid coexists with the two solids simultaneously; Although the low-density solid melts to a high-density flui d, this transition is found to be metastable with respect to the transforma tion into a high-density solid. This is evidence that short-range tetrahedr al attractive forces are not in and of themselves sufficient to explain the anomalous melting of water. Our results indicate that vapor-liquid equilib rium for the model is preempted by solidification. Monte Carlo simulation r esults 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].