The research in this thesis would have taken far longer to complete without the encouragement from many others. It is a delight to acknowledge those who have supported me over the last three years.
I would like to thank my supervisor, Dr. Richard Needs, for his guidance and relaxed, thoughtful insight. I thank Dr. Guna Rajagopal for his hospitality and for his remarkable demonstrations of the process known as ``negotiation''.
This work was supported by an EPSRC studentship and substantial allocations of supercomputer time, without either of which this work would not have been possible.
I am particularly thankful for the help and advice of Dr. Randolph Hood and Dr. Andrew Williamson during my first few months at the Cavendish, without whom the learning curve would have been very much steeper.
The days would have passed far more slowly without the support of my friends, both at the Cavendish and around Cambridge, whom I thank for putting up with my idiosyncrasies and for providing such a rich source of conversation, education and entertainment.
I am indebted to those at the Cavendish Laboratory who welcomed me at such short notice, and to the many members of staff at Bath University who encouraged me on this journey.
Finally, I wish to thank my parents for their love and encouragement, without whom I would never have enjoyed so many opportunities.