Developing Water and Methane Potentials for MD Simulations of Methane Clathrate Hydrate
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The aim of this thesis is to develop a set of intermolecular potentials that enable the study of nucleation or decomposition of methane hydrates. The potentials are developed for water, methane, and the water-methane pair by fitting to SAPT(DFT) reference energies. The first set of potentials developed differ from recent polarisable models in that they have rank 4 ISA multipoles, rank 3 anisotropic polarisabilities, rank 3 isotropic dispersion, and anisotropic exchange-repulsion terms. These potentials are validated based on the structures and energies of small clusters and second virial coefficients. The potentials are then significantly simplified for use in MD simulations with DL_POLY4. Simplifying the methane potential makes it difficult to fit simultaneously the global minimum dimer and a set of randomly generated dimers used as reference energies. Several methods are tested to account for polarisation in water within the limitations of DL_POLY and it is found that for MD simulations good results can be attained by increasing the charge values to match the multipole moments of a water molecule in a dielectric. Simulations are carried out for liquid water, ice Ih, and methane gas to validate the new models. The models developed are compared in MD simulations with TIP4P/Ice and the United Atom Methane (UAM) model in simulations of sI methane clathrate; both under stable conditions and while undergoing decomposition at different temperatures. It is found that the melting behaviour differs according the methane- and water-methane interactions; the behaviour of methane under clathrate decomposition using either methane model is discussed.
AuthorsGilmore, Rory Alan John
- Theses