Hydrogen fluoride phase behavior and molecular structure: A QMÕMM potential model approach

A molecular-orbital derived polarizable potential function is developed to model liquid and supercritical fluid hydrogen fluoride. The model is based on a novel application of a combined quantum-mechanical and molecular-mechanical ~QM/MM! approach, which treats molecular polarization by a semiempirical method. Two geometrical models are examined, differing in the intramolecular bond length for hydrogen fluoride to match values commonly seen in other empirical models. One QM/MM parameter is fit for each model to reproduce the experimental density at one liquid-phase state condition. The models are examined at this state and at one supercritical state condition. Results for the density, radial distribution function, and average molecular dipole moment are considered in comparison to experiment. Also vapor-liquid coexistence data are evaluated, including saturation densities, heat of vaporization, and vapor pressure. Both models perform well in describing the densities, but are no better than other molecular models in characterizing the vapor-liquid critical point, the heat of vaporization, and the vapor pressure. The QM/MM models are slightly better than others in describing the radial distribution functions, although it is clear that this QM polarization model can be further improved. The present study further demonstrates that a QM-based polarization model is a viable alternative to model polar fluids with strong intermolecular interactions. © 2003 American Institute of Physics. @DOI: 10.1063/1.1607919#