Non-additive intermolecular forces from the spectroscopy of Van der Waals trimers: A comparison of Ar2–HF and Ar2–HCl, including H/D isotope effects

Abstract

Variational calculations of vibrational energies, rotational constants and angular expectation values are carried out for the trimers Ar2–HCl, Ar2–DCl, Ar2–HF and Ar2–DF. Calculations are performed on pairwise additive potential energy surfaces and on surfaces including a variety of non-additive contributions. Attention is focused on the bending levels corresponding to hindered internal rotation of the HX molecule in the complex, several of which have been observed by high-resolution spectroscopy. The results confirm that it is crucial to include dispersion, induction and short-range effects when modelling the non-additive forces in molecular systems. It is found that the model of non-additive forces previously proposed by Ernesti and Hutson [Phys. Rev. A 51, 239 (1995)] works well for the bending bands of Ar2–HCl, Ar2–DCl and Ar2–DF as well as Ar2–HF. In addition, a new distributed model of the non-additive dispersion energy is proposed, in which the triple-dipole energy is partitioned between two anisotropic sites in the HX molecule.