Calculations of vibrational energies and rotational constants are carried out for the van der Waals trimer ${\mathrm{Ar}}_2$-HF. The calculations include all five intermolecular degrees of freedom. The different intramolecular vibrational states v of the HF molecule are separated out adiabatically, so that the calculations are carried out on effective intermolecular potentials for each HF vibrational state separately. Calculations are performed both on pairwise-additive potentials, derived from the well-known Ar-Ar and Ar-HF potentials, and on nonadditive potentials, incorporating various different contributions to the three-body forces. The results are compared with experimental results from high-resolution spectroscopy, and provide detailed information on the anisotropy of the nonadditive intermolecular forces. As in previous work on ${\mathrm{Ar}}_2$-HCl, it is found that a very important nonadditive term arises from the interaction between the permanent multipoles of the HF molecule and the exchange quadrupole caused by distortion of the two Ar atoms as they overlap. An improved model of this term is described.

• Received 19 July 1994

DOI:https://doi.org/10.1103/PhysRevA.51.239