Tribello, G.A.; (2007) Structure and energy relationships in ice and crystalline hydrates. Doctoral thesis , University of London.

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Abstract

Computer simulations of the various phases of ice have been carried out using potential methods and density functional theory. Plane wave DFT and subsequent Wannier transformations of the Kohn-Sham orbitals were used to obtain highly localised orbitals, which were treated as molecular orbitals in the calculation of molecular multipoles. Using these multipoles it has been shown that the energy differences, calculated using DFT, between different proton topologies of ice VII and Ih are reproduced when the interaction electrostatic potential energy is calculated up to terms in (1/r6) and thus that the driving force for proton ordering is electro static. Armed with this knowledge, successful blind predictions, which have been experimentally verified, of the proton ordered forms of ice V and XII (ices XIII and XIV respectively) have been made using plane wave DFT. The recently developed TIP6P potential has been modified so as to reproduce the correct structure for ice XI, the proton ordered form of ice Ih, and to reproduce the DFT energy differences between different hydrogen bonding topologies. Total energy calculations, using this potential, show that the surface energy depends strongly on the hydrogen bond topology exposed at the surface. In particular surfaces on which under-coordinated protons are clustered have high energies. Monte Carlo calculations have shown that the hydrogen bond topology adopted by ice, both at the surface and in the bulk, depends on the temperature. A comparison of the structures that are possible to make out of silica and ice has been undertaken in the hope that new ice and silica phases can be identified. This comparison is possible because both silica and water form the backbones of 4-connected nets. DFT calculations have shown that the energy maps of the various four connected nets are very similar for both structures, with any differences arising because of the greater flexibility of the O-Si-O angle in silica. Furthermore, this analysis has highlighted a number of potential new ice phases and led to the proposal of a synthetic route to a new clathrate based on the zeolite framework SGT.

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