Nonadditive intermolecular forces from the spectroscopy of van der Waals trimers: Calculations on Ar2–HCl

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Abstract
The spectra of van der Waals trimers formed from two atoms and a diatomic molecule are investigated. A computational method for calculating vibrational energies, rotational constants and angular expectation values for such complexes is developed and applied to the Ar2–HCl complex. All five low‐frequency modes of the trimer are included. The pair potentials for Ar–Ar and Ar–HCl interactions are well known, and calculations are performed on Ar2–HCl potentials obtained from them assuming pairwise additivity. The calculations are compared with experimental results obtained from microwave and far‐infrared spectroscopy. Substantial discrepancies between experiment and theory are found, and are attributed to the effects of nonadditive interactions. Several different contributions to the nonadditive interactions are investigated: dispersion forces, electrostatic induction forces, and exchange overlap forces are all found to be significant, but not large enough to explain the discrepancies. Exchange multipole forces, arising from the interaction between overlap‐induced multipoles on the Ar atoms and the permanent multipoles on the HCl molecule, are found to be important, and to improve the agreement between experiment and theory substantially. It seems likely that it will be possible to obtain detailed information on nonadditive interactions from the spectroscopy of van der Waals trimers.