Ab initio investigation of hydrogen bonding and network structure in a supercooled model of water

The hydrogen bond (HB) and network structure in a large periodic model of 340 water molecules are investigated by ab initio methods. This model has a density of 1.00 g/cc, very small distortions of O-H bond length and H-O-H bond angle, and the peak positions in the radial distribution functions in close agreement with experiment. The 340 molecules can be classified into four groups according to the number of HBs quantitatively determined by the bond-order values. The percentages of water molecules with two, three, four, and five HBs are, respectively, 3.2%, 10.3%, 85%, and 1.5%, with an average HB number per H2O of 3.85, which clearly supports the tetrahedral network description for this model. The O K edges of the x-ray absorption near edge structure of every water molecule are calculated by the ab initio supercell orthogonalized linear combination of the atomic-orbitals method. Statistical analysis of the calculated O K-edge spectra shows that the experimentally observed pre-peak originates predominately from H2O molecules with two HBs, and that there exist large fluctuations in the spectral features of individual water molecules within a given set.