This dissertation describes work done between October 1995 and June 1998 in the Theory of Condensed Matter group at the Cavendish Laboratory, Cambridge, under the supervision of Dr. M. C. Payne.
Except where stated otherwise, this dissertation is the result of my own
work and contains nothing which is the outcome of work done in
collaboration. This dissertation has not been submitted in whole or in part
for any degree or diploma at this or any other university.