Calculation of the far-infrared spectra for (HF)2, (HCL)2 and (HBr)2

Calculations of far-infrared spectra for the weakly bound complexes (HF) 2 , (HCl) 2 and (HBr) 2 are reported. For (HF) 2 and (HCl) 2 , potentials fitted to ab initio data are used. Spectra are also predicted using simple classical potentials. The method involves diagonalising the Hamiltonian for the dimer with the monomer and intermolecular bond lengths held fixed. The approach gives very good agreement with energy levels calculated with the close coupling method, particularly for (HCl) 2 . Furthermore, the simple electrostatic potential gives a spectrum for (HCl) 2 that agrees surprisingly well with that obtained from the ab initio potential energy surface.