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 fact that hydrogen bonds are highly directional and favor the formation of locally open structures. The resulting analytical equation of state reproduces 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 present work from a simple microscopic model. Furthermore, by altering the parameters which control the geometric constraints on bonding, the model is able to exhibit the two thermodynamically consistent scenarios that can explain the observed behavior of supercooled liquid water, namely the two-critical-point and singularity-free scenarios. This suggests that the two scenarios are closely related through subtle features of the hydrogen-bond geometry.