Nonadditive intermolecular forces in Ar-n-HF van der Waals clusters: Effects on the HF vibrational frequency shift

The effects of nonadditive forces on Ar-n-HF van der Waals clusters are inv estigated for n = 2, 3, 4, and 12. The pair potentials operating in these s ystems are accurately known. Earlier models of nonadditive forces in Ar-2-H F, including nonadditive dispersion, induction, and overlap distortion, are generalized to handle clusters of arbitrary size. Calculations of vibratio nal frequency shifts (redshifts) are then performed and compared with exper iment. The geometries of the clusters are first optimized by simulated anne aling; the Ar n cage is then held fixed, and the resulting five-dimensional Schrodinger equation is solved for the hindered rotational and translation al motion of the HF molecule in the field of the Ar atoms. The nonadditive potentials are found to account remarkably well for the observed frequency shifts. (C) 1999 American Institute of Physics. [S0021-9606(99)00142-7].