AN ABINITIO CLOSE-COUPLING CALCULATION OF THE LOWER VIBRATIONAL ENERGIES OF THE HF DIMER

We have previously determined an analytical ab initio six-dimensional potential energy surface for the HF dimer, and in the present paper we use this potential with the HF bond lengths held fixed in a full (four-dimensional) close-coupling calculation to determine the energies of 18 low-lying vibrational states. These vibrational states involve the intermolecular stretch nu-4, the transbend tunneling vibration nu-5, and the torsion nu-6. As well as determining tunneling energies up to 5-nu-5 = 341 cm-1, we determine nu-4 = 122 cm-1, 2-nu-4 = 236 cm-1, 3-nu-4 = 342 cm-1, nu-6 = 378 cm-1 and nu-4 + nu-6 = 491 cm-1, together with the tunneling splittings in all these states. A strong perturbation between 4-nu-5 and nu-4 + 2-nu-5 is found for this ab initio potential. Making allowance for the HF stretching zero-point energy, we determine the dissociation energy D0 as 1012 cm-1. We hope that these energies provide benchmark vibrational energies for this ab initio surface for the purpose of testing the accuracy of other more-approximate calculations. We also hope that our results will be of help in assigning experimental spectra and in identifying perturbations.