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The temperature-dependent hydrogen-bond geometry in liquid water is studied by x-ray Compton scattering using synchrotron radiation combined with density functional theory analysis. Systematic changes, related to the weakening of hydrogen bonding, are observed in the shape of the Compton profile upon increasing the temperature. Using model calculations and published distribution functions of hydrogen-bond geometries obtained from a NMR study we find a significant correlation between the hydrogen-bond length and angle. This imposes a new constraint on the possible local structure distributions in liquid water. In particular, the angular distortions of the short hydrogen bonds are significantly restricted.

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We have carried out high-resolution Compton scattering measurements from two single crystals of Be ͓͑00.1͔ and ͓11.0͔͒, together with highly accurate all-electron first-principles computations of the profiles within the band theory framework. The Compton data were collected using a newly constructed crystal spectrometer at a record momentum resolution varying between 0.023 and 0.032 a.u. ͑full width at half maximum͒ with 8 keV x rays from a synchrotron source. Although the overall shapes of the measured and computed spectra are in good accord, the fine structure in the data shows significant discrepancies with respect to the local density approximation based theoretical predictions. In particular, several features in the observed spectra are substantially broader than the computations, indicating the importance of electron correlations and other effects in analyzing momentum densities and Compton profiles from solids. ͓S0163-1829͑96͒02132-7͔