The work described in this dissertation was carried out in the Department of Theoretical Chemistry at the University of Cambridge between October 2002 and January 2006. The contents are the work of the author and contain nothing which is the outcome of work done in collaboration with others, except as specified in the text and Acknowledgements, and have not previously been submitted for any degree or qualification at another institution. The number of words does not exceed 60 000.

Semen A. Trygubenko
Cambridge, January 2006