In this thesis, the accuracy of model potentials developed for a variety of weakly bound systems is assessed. Stationary points on the potential energy surfaces are compared with ab initio results for both water and hydrogen fluoride clusters. The second pressure virial coefficient has also been calculated and compared with experimental data where available. The inclusion of quantum corrections, especially those associated with the rotational degrees of freedom, proves to be essential for an accurate description of this quantity. For small water clusters we have made a detailed comparison of many-body energies calculated using ab initio methods and two new model potentials. It is shown that induction energy is an important many-body term, so that two-body potentials which account for it correctly can be applied unchanged to many-body systems with good results. New models have been developed to study protonated water clusters and the validity of the rigid-body approximation is discussed. We show that the minimum energy structures are consistent with those located using density functional theory methods, and these are expected to compare well with correlated ab initio calculations. Here, the use of a model potential as a precursor to performing more accurate calculations has been demonstrated to be an economical way to proceed in the precise and systematic characterization of a system. The suitability of the Tang-Toennies damping functions commonly used to account for the short-range effects of charge-overlap on the dispersion energy has been examined. The argon dimer was chosen as a prototype system on which symmetry-adapted and intermolecular perturbation theory calculations were performed and compared. We show that such functions fit the data better when exchange as well as charge-overlap effects are considered. We also construct new damping functions and these provide a more accurate, yet still quite compact, description of the dispersion energy.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:604126 |
| Date | January 1999 |
| Creators | Hodges, M. P. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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