Liquid hydrogen fluoride is a simple but interesting system for studies of the influence of hydrogen bonds on physical properties. We have performed ab initio molecular dynamics simulations of HF at several thermodynamic states, where we examine the microscopic structure of the liquid as well as its static and dynamic properties. The results obtained show good agreement with well established data, and, moreover, we were able to show significant changes within the structure depending on the system's temperature and density.

The gas phase of hydrogen fluoride has been investigated by neutron diffraction experiments at three different particle densities. All investigated states are within the liquid-gas coexistence region of hydrogen fluoride. From the obtained diffraction data we deduced information about the local structure of the gas phase, which consists of small agglomerates. This has been expected as liquid hydrogen fluoride forms the strongest hydrogen bonds known. Molecular dynamics simulations with a modified potential have been carried out for all experimentally investigated states. The results confirmed that the size of the formed agglomerates in the gas phase is growing with increasing density of the gas phase.

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High pressure neutron diffraction experiments on a mixture of CH4 and CD4 as well as on pure deuterated methane CD4 were performed at three supercritical states. The structure factor was determined at three different densities at a temperature of T = 370K. The intra- and intermolecular structure was determined from the structure factor. The results from pure CD4 are in very good agreement with former neutron diffraction experiments. The experiments on the isotopic mixture allowed the direct determination of the pair correlation function of the molecular centres. The results of the diffraction experiments are presented in this paper.

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