This dissertation describes work undertaken between October 1996 and August 1999 in the Theory of Condensed Matter group at the Cavendish Laboratory, Cambridge, under the supervision of Dr. Richard Needs.
Except where reference is made to the work of others, this
dissertation is the result of my own work and contains nothing which is
the outcome of work done in collaboration. This thesis has not been
submitted in whole or in part for any degree or diploma at this or any
other university.