A single-bond approach to orientation-dependent interactions and its implications for liquid water

A simple model of an associating fluid is proposed that accounts for the fa ct that hydrogen bonds are highly directional and favor the formation of lo cally open structures. The resulting analytical equation of state reproduce s the distinguishing thermodynamic features of liquid water. In contrast to previous models in which the relationship between bonding and bulk density is assumed a priori, the extent of hydrogen bonding is derived in the pres ent work from a simple microscopic model. Furthermore, by altering the para meters which control the geometric constraints on bonding, the model is abl e to exhibit the two thermodynamically consistent scenarios that can explai n the observed behavior of supercooled liquid water, namely the two-critica l-point and singularity-free scenarios. This suggests that the two scenario s are closely related through subtle features of the hydrogen-bond geometry . (C) 1999 American Institute of Physics. [S0021-9606(99)50230-4].