Firstly, my supervisors Andrew Briggs and David Pettifor, who have allowed me the freedom to pursue what I found interesting, while being happy to provide advice, guidance and help wherever needed. I have greatly enjoyed my time working with them. I would also like to thank my new employer, Mike Gillan, for allowing me to work on my thesis as well as on CONQUEST, and for his insights into the subject.
It is often the case that while the supervisors provide overall guidance and ideas, post docs give a more detailed level of help. I have been extremely fortunate in working closely with two post docs in particular: Chris Goringe and Andrew Horsfield. They have both helped me enormously over the last three years, while being very patient with my incessant questions. Chris has helped me more with the systems I have modelled, while Andrew has taught me more about the methods I have used, but there has also been a great deal of overlap.
As well as theoretical work, this thesis revolves around experiments, and those experimentalists I have worked with deserve thanks for their patience and enthusiasm. First and foremost of these is James Owen, whose thesis should be consulted for details of any of the experiments mentioned herein. Our collaboration has proved fruitful, and our friendship has enriched my time in Oxford. Other people in the lab with whom I have worked closely include Ilan Goldfarb, Kazushi Miki and Holger Nörenberg.
The Materials Modelling Laboratory is a superb environment in which to work, and there are many people there who have helped me at one time or another. Neil Long performs wonders in maintaining the computers, and has been very helpful and supportive during the writing of this thesis. Other people who have helped me include Alex Bratkovsky, Mike Fearn, Hideaki Fujitani and Tony Paxton.
Many friends have contributed to keeping me and Erica sane and happy during this time, and I should like to thank Mike and Hester, Steve and Heather, John and Claire, Chris and Su; as well as these friends around the country, there have been many friends in the lab and around Oxford who have contributed to the profits of the Royal Oak with us: James, Catherine, Adam, Adrian, Chris, Paul, Rob, James and Joanna.
Finally I would like to acknowledge my family: my grandmother, for her love and friendship over many years, for teaching me the piano and sharing the Blüthner; my father, for showing me how to do physics properly, and for willing help during the writing of this thesis; and Erica, whose love and support has made all this possible.