The Cochrane Library

In recent years, first principles investigations have been applied successfully to the cal-culation of the electronic and geometric structures of bulk solids. In this thesis, I have concentrated on extending this territory toward the realm of molecules and liquids. As a first step in this direction, I describe calculations of hydrogen adsorption on the classic (111)-(7x7) reconstructed surface of Si. The myriad of dangling bond sites on this extremely complex surface offer a unique "laboratory " in which to study atom-surface interactions. As a next step, I then apply the same theoretical foundation to study the molecular complex (H20)6-. Such clusters of water molecules can be created in fine-mists but, other than their existence, very little is known about them either experimentally or theoretically. By combining state-of-the-art quantum chemical and first-principles density functional techniques, this study not only results in a wide range of new predictions but also helps establish the validity of the first-principles approach to the case of non-solid environments. Finally, I introduce the concept of a molecular pseudopotential in order to investigate, in a tractable first-principles fash