The research presented in this thesis may be divided into three areas: transition metal catalysis, graphical user interfaces, and the derivation and application of effective core potentials;Transition metal catalysis. The titanium catalyzed hydrosilation reaction has been examined in detail to determine a possible minimum energy reaction path. Two reactions and three catalysts were considered. In addition to a model system consisting of the reaction of silane with ethylene, the simplest known experimental reaction involving trichlorosilane and ethylene was also examined. In addition to a model catalyst of TiH2, the experimental catalysts TiCl2, and Ti(C5H5) 2 were considered. In all reactions studied the catalyst had a dramatic effect on the reaction system changing the overall reaction barrier from over 50 kcal/mol without the catalyst to a barrierless process with the catalyst. In addition to the overall reaction energies, several low energy intermediate structures were predicted which might be experimentally observable;Graphical user interfaces. A description is given of a new graphical user interface for the GAMESS program. This interface includes many features useful for interpreting complex wavefunctions and reaction systems. These include the ability to animate reaction paths and normal modes of vibration, as well as the ability to view molecular orbitals, total electron densities, molecular electrostatic potentials, and density differences;Effective core potentials. The derivation of an enhanced method for the computation of integrals involving effective core potentials is presented and has been implemented in the electronic structure code GAMESS. This method has helped produce a large reduction in the computational cost of ECPs. It has also enabled the implementation of analytical second derivatives. The new method is also applied to the determination of the minimum energy structures of Si8C12, Ge8C12 and Sn8C12 which are main group analogs of the Ti8 C12 compounds (known as metcars). Relative energies, geometries, and vibrational frequencies are reported for several novel structures.