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Molecular dynamics simulation techniques have been used to investigate behaviour of sorbed CH, and C,H, in zeolite ZSM-5. Our calculations allowed for framework in addition to molecular motions. Diffusion trajectories were obtained for the sorbed molecules. Simulated diffusion coefficients were in acceptable agreement with experiment.

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The results of first principles calculations on H-silsesquioxanes (i.e., (HSiO 3/2 ) n with n ) 4, 6, 8, 10, 12, 14, and 16) are reported here. Double numeric basis sets and local and nonlocal density approximations to density functional theory are employed for calculations. It is shown that use of the nonlocal density approximation is required for the reliable prediction of the most stable isomer for silsesquioxanes. Furthermore, a progression of the preferred building unit with the increase in size of the T cage is revealed. The smaller T cages prefer four-and five-member rings while the larger cages are found to prefer four-and six-member rings. Analysis of the energy of the hydrolysis reaction, binding energy, and fragmentation paths finds the relative stability of the silsesquioxane cages containing four-, five-, and six-member rings in agreement with experimental observations. For the (HSiO 3/2 ) 16 cage, the calculated results predict the stability of the D 2d -6 4 5 0 4 6 configuration over the D 4d -6 0 5 8 4 2 configuration in contradiction to suggestions based on 29 Si NMR measurements. We find a consistent picture for the highest occupied molecular orbitals (HOMOs) of all silsesquioxanes considered showing them to be composed of (lone-pair) oxygen p-type atomic orbitals. On the other hand, the lowest unoccupied molecular orbitals (LUMOs) show size dependence in their composition which appears to cause the presence of a state in the HOMO-LUMO gap for higher silsesquioxane cages. Density of states plots and analysis of molecular orbitals reveal this state to be due to the terminal hydrogens bonded to silicon atoms.

Periodic density functional calculations (DFT) on bridging hydroxyl groups in the zeolite faujasite are performed. It is shown that force field calculations as presently parametrized are not able to reproduce the correct energetical ordering for these groups. Embedding not only gives the right ordering but also agrees well with the periodic calculations for geometries. OH stretching frequencies can be obtained in very good agreement with experiment by periodic DFT calculations in particular if anharmonic corrections are included. The same functionals used for the periodic calculations have been employed in calculations on model clusters, and it is shown that clusters provide a qualitative as opposed to a quantitative description of these systems.

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A combined Monte Carlo-simulated annealing approach has been developed to predict the location and orientation adopted by organic molecules within a zeolite host. Periodic boundary conditions were used throughout the simulations in order to permit a full treatment of organic-organic interactions within a densely packed system. This approach has been used to investigate the relationship between zeolite products and the organic template molecules used in their synthesis. The results are shown to reproduce experimentally determined locations accurately, including important disordering effects. Previous experimental work has shown that some template molecules can effect the synthesis of more than one type of zeolite framework when experimental conditions are varied and also that many zeolites can be synthesised by an apparently diverse range of templates. Our results show that both these observations can still be rationalised via the 'templating theory' of zeolite synthesis. A mechanism for faulting is proposed which is shown to be consistent for NU-86 and the industrially important zeolite beta.