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Title: Density methods in quantum chemistry
Author: Gilbert, A. T. B.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2001
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Density functional theory (DFT) has become a central aspect of quantum chemistry and provides the mainstay of chemical calculations. The advantage of DFT methods lies in their relatively inexpensive computational cost and their dealing with an experimentally tangible quantity, i.e. the density. The chief drawback is its lack of well-defined path from approximation to exactitude. Consequently many models and approaches have emerged, and been enthusiastically advocated, often with little more justification than "it works". This thesis begins with an overview of traditional quantum chemical theory and methods, and places DFT within this framework. In chapters 5 and 6 new model systems are presented, and novel methods to derive exchange functionals that are exact for these model systems are developed. By taking this approach, rather than the more pragmatic one of data fitting, the successes and failures can be traced to the underlying soundness of the model and/or the method used in the derivation. In the development of these functionals some difficulties were encountered with convergence of the self-consistent field calculations. These problems are addressed in chapter 7. The observation that a molecular density is very close to that given by the superposition of its constituent atoms, leads naturally to the idea of modelling a density by a sum of nuclear centred, spherically symmetric 'Stewart Atoms'. However, attempts at constructing Stewart atoms in the past have been thwarted by slow basis set convergence. In chapter 8 we overview the work that has been undertaken on Stewart atoms and present several formalisms in which the theory has been developed. Chapter 9 deals with the problem of constructing accurate representations of Stewart atoms. Several different approaches are considered and the most accurate is determined. Applications of the Stewart atoms are considered in chapter 10.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available