VAPOR-LIQUID-EQUILIBRIA AND HEAT-EFFECTS OF HYDROGEN-FLUORIDE FROM MOLECULAR SIMULATION

The vapor-liquid coexistence densities, vapor pressure, and heat of va porization of hydrogen fluoride (HF) is calculated via Monte Carlo sim ulation from three intermolecular potential models that are found in t he literature. The first is a pure pair potential based solely on ab i nitio data, the second is a semi-empirical pair potential which uses a n ab initio derived surface fitted with dimer spectroscopic data, and the third is an effective pair potential that was fit to experimental data for the condensed phase. As expected, the effective potential rep roduces the saturated liquid densities more accurately than the others do, while all the potential models predict the wrong slope and curvat ure in the vapor pressure curve. The inability to reproduce the vapor pressure dependence on temperature is connected to the models' poor pr ediction of the heat of vaporization at temperatures below 400 K. A bi asing algorithm is introduced to study the superheated-vapor heat capa city, density, association number, and oligomer distribution along thr ee low-pressure isobars using both the semi-empirical and effective pa ir potentials. It is found that both these potential models do predict a peak in the heat capacity, however, they are at cooler temperatures and only about half the magnitude relative to the experiment. When co mparing the potential models to each other, it is found that the semi- empirical pair potential predicts the onset of near-ideal gas conditio ns at about 30 K cooler than the effective pair potential. Additionall y, the percentage of ring oligomers predicted by both models is consid erable at all but the highest temperatures. Both models also agree tha t the monomer and cyclic tetramer are the two most important species a t the nonideal states. (C) 1998 American Institute of Physics. [S0021- 9606(98)50134-1].