| Computer simulation has become a useful tool for studying chemical reactions and spectroscopy. However, the reliable application of computational modeling to large and complex reactive systems involving electronically excited states is still limited. Two of the most important challenges in these applications are the accurate and efficient first-principles calculation of coupled ground- and excited-state potential energy surfaces (PESs), and the modeling of such PESs. For the former challenge, accurate electronic structure methods including static and dynamic electron correlation are often too costly for complex systems, while more affordable methods such as density functional theory (DFT) at their current stage of development are still not satisfactorily accurate for many such systems. For the latter challenge, the high dimensionality and complicated topography of the PESs of complex systems make it difficult to choose a model representation.;This thesis presents several responses to these challenges: (a) Improvements to time-dependent DFT are made to provide better accuracy for excited states and for PESs. (b) A diabatization scheme is developed for more accurate and efficient modeling of coupled PESs. (c) Simple models are presented for efficient simulation of the band shape of the electronic spectroscopy of complex molecules. (d) State-of-the-art methods are applied to simulate the electronic spectrum, and to build the PESs for the photochemistry, of a complex reactive system, thioanisole. |
