Exact full-dimensional bound state calculations for (HF)2, (DF)2, and HFDF

Detailed results of the converged full-dimensional 6D quantum calculations of the vibrational levels of (HF)2, (DF)2, and HFDF, for total angular momentum J=0, are presented. The ab initio 6D potential energy surface by Quack and Suhm was employed. This study provides a comprehensive description of the bound state properties of the HF dimer and its isotopomers, including their dissociation energies, frequencies of the intermolecular vibrations, tunneling splittings, and extent of wave function delocalization. Quantum number assignment of the calculated eigenstates by plotting different cuts through the wave functions worked rather well for (HF)2, but proved to be much harder for (DF)2 and HFDF, indicating stronger vibrational mode mixing in these species. The ground-state tunneling splitting for the HF dimer from our exact 6D calculations, 0.44 cm−1, is very close to that from a previous 4D rigid-rotor calculation, 0.48 cm−1 J. Chem. Phys. 99, 6624 (1993). This is in disagreement with the result of a recent 6D bound state calculation for (HF)2 by Necoechea and Truhlar, which gave a ground-state tunneling splitting a factor of 3.7 times larger than the 4D result.