In this thesis, the application of the computational hybrid method ONIOM is explored in relation to locating critical points .on a reaction potential energy surface. ONIOM is a very generalized hybrid method, which integrates molecular orbital and molecular mechanics methods to treat the' molecule with different levels of theory, such that the reactive site is described by an accurate method whilst the full molecule is included at a lower level of theory. .This thesis demonstrates the suitability of the ONIOM method for calculating the excited states of large molecules and locating important regions of ultra-fast' radiationless decay to the ground state, where previously it has almost exclusively been used for ground state study. The critical points locatep using the ONIOM method are validated by comparison to the non-ONIOM calculations on the full molecule. The location of transition structures is important in any mechanistic study since these are energy barriers between reactant and product wells. This work highlights a previously unknown problem with ONIOM transition state optimizations and explains its origin by examining the ONIOM frequencies. The transition states of two ground state Diels-Alder reactions are examined demonstrating the reliability and usefulness of the ONIOM method. The application of diagnostic tests to assess the ONIOM method is explored and the results of these confirmed by comparison to the full non-ONIOM calculations. The work described in $is thesis letl to code developments related to both conical intersection and transition structure searches using the ONIOM method. It is evident from our work that the ONIOM method can offer significant computational savings and is a suitable method for characterization of points on the potential energy surface for both ground and excited state reaction path calculations'.