The following chapter outlines the theoretical framework underlying the calculations on which the work in this thesis is based. A number of approximations must be made in order to solve the quantum mechanical equations governing all but the smallest of systems. These approximations are described and justified.
In order to analyse the results of ab initio calculations, it is
useful to have a measure of local atomic properties such as ionic
charges and bond populations. Unfortunately, the methods used for the
electronic structure calculations, based on the use of a plane wave
basis set to represent the electronic eigenstates, do not provide
natural definitions of these quantities. Chapter 
details a method by which these local properties may be calculated
by projection of the quantum mechanical eigenstates,
resulting from an ab initio calculation, onto a localised atomic
basis set. The implementation of this technique is described in
Appendix . Applications of this population analysis to
molecular and bulk systems are discussed. These include the use of
ionic charges and bond populations as measures of ionicity and
covalency of bonding, and bond population as a measure of bond
modulus.
Chapter describes the use of the techniques
detailed in the previous chapters in a study of the acetylcholine
molecule. This molecule has been widely studied using semi-empirical
calculations and conventional experimental techniques due to its
central rôle in neurotransmission. Thus, it is a good subject for the
validation of an ab initio approach.
The cytochromes P450, an important superfamily of enzymes which are of
current interest in pharmacology and toxicology, are investigated in
Chapter . These enzymes are responsible for the metabolism
of many endogenous and xenobiotic compounds, including drugs. The
interactions between ligand molecules and the active site of these
enzymes are modelled and the results compared with experiment. The use
of `computational experiments' is demonstrated in an exploration of
the rôle of a water molecule at the active site of the enzyme in the
inhibition of the catalytic reaction.
Finally, Chapter summarises the contents of this
thesis and suggests some possible directions for future research.
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