Integral equation study of liquid-hydrogen fluoride

Liquid hydrogen fluoride is a well-known hydrogen bonded substance, in many aspects related to liquid water, and for which a wide variety of interacti on models have recently been proposed. We have studied two of;these models by means of a reference hypernetted chain equation in order to assess the a bility of this latter approach to describe the properties of this highly as sociative system. Our calculations, when compared with molecular dynamic re sults, show that the integral equation reproduces quantitatively both the s tructure and the thermodynamics of liquid hydrogen fluoride over a wide ran ge of thermodynamic states. However, the integral equation approach is appa rently unable to produce estimates for the phase diagram since the low-dens ity (gas phase) side of the binodal curve lies inside the nonsolution regio n of the equation. This failure can be understood as the result of the inab ility of standard integral equation theories to account for the behavior of low density strongly associative systems like highly charged electrolytes or, in this case, the gaseous phase of hydrogen fluoride. (C) 2001 American Institute of Physics.