MODELING WEAK INTERACTION ELEMENTS AFFECTING THE STRUCTURES AND VIBRATIONAL RED-SHIFTS OF ARNHF CLUSTERS (N=1 TO INFINITY)

It is shown that a concise model potential can well account for the gr ound vibrational state rotational constants of Ar1-4HF clusters, the H F vibrational frequency shift of Ar1-4HF clusters, and the vibrational frequency shift of HF in an argon matrix. The model potential explici tly incorporates direct intermolecular polarization, which is a signif icant contributor to the stabilities of the clusters. Direct polarizat ion produces an Ar-HF interaction that is different for HF in its grou nd vibrational state and its first excited vibrational state. That dif ference in interaction accounts for a sizable share of the HF red-shif t for Ar clusters of all sizes. The calculations include full treatmen t of vibrational dynamics via quantum Monte Carlo for the ArnHF cluste rs with n = 1-12, and these show small effects of weak mode vibrations on the HF rid-shift. Calculations of the fully optimized equilibrium structures for n = 1-176 clusters plus calculations with optimized lat tice structures of four, five, six, and seven solvation shells (throug h n > 2000) were used for extrapolation to an infinite cluster limit v alue for the HF red-shift. The second solvation shell is found to be n oticeably important in the matrix red-shift. Three-body dispersion in the model potential has a direct effect on the red-shifts in large clu sters, but the indirect effect via the influence on Ar-Ar separation i s small. (C) 1998 American Institute of Physics.