Energy surfaces and electronic properties of hydrogen fluoride

Results of first-principles calculations on a single hydrogen fluoride molecule as well as on poly- meric hydrogen fluoride are reported. The results are compared with other theoretical calculations as well as with experiments and the agreement is found to be good. A model for solitonic excitations is proposed and examined. The parameters entering the model are extracted from the first-principles calculations. The model represents the first attempt to include electronic interactions in the discussion of solitonic excitations in hydrogen-bonded polymers. It predicts the soliton to be a sharp kink and that the soliton induces electronic states slightly outside the energy bands of the unperturbed system. The first-principles results are also used in investigating the frozen optical stretch phonons at the zone center within both a harmonic and an anharmonic approximation. The latter is found to give rise to an extra mode compared with the former. Finally, we compare our total energies with those predicted by various proposed model potentials for describing dynamical properties of the gas and liquid phases. Some discrepancies are found, and it is argued that variations in the molecular bond length as well as electronic interactions are to be included in realistic model potentials.