Basis set dependence of the structure and properties of liquid hydrogen fluoride

An intermolecular potential function for the hydrogen fluoride dimer has been obtained from ab initio molecular orbital calculations with a minimal basis set (STO-3G). Dimerization energies for 285 orientations of (HF)2 selected using the enery distributed random geometries method were fit to a 12–6–3–1 potential with standard deviations of ∼0.3 kcal/mol. The function was used in a Monte Carlo simulation of liquid hydrogen fluoride at 0 °C. Structural and thermodynamic results are compared with those based on an earlier potential derived from double zeta basis set (6-31G) calculations. The structural predictions are affected by the local density which is too high from the 6-31G potential. Significant improvement is obtained with the STO-3G function. A detailed analysis of the structure of liquid HF is presented including coordination number and energy distribution functions. Each HF monomer has an average of two hydrogen bonded neighbors with an additional five peripheral contacts within 3.6 Å. The hydrogen bonded monomers form winding chains with significantly bent hydrogens bonds in contrast to the linear hydrogen bonds found in the solid and in the gas phase dimer. As in the case for water, it is apparent that reasonable intermolecular potential functions for hydrogen bonded dimers can be obtained from minimal basis set calculations. In the absence of CI level calculations, this appears to be the method of choice.