Development of nonorthogonal wavefunction theories and application to multistate reaction processes.

Many prominent areas of technological development rely on exploiting the photochemical response of molecules. An application of particular interest is the control of molecular switches through a combination of different external stimuli. However, despite significant advances in theoretical approaches and numerous cases of successful application of theory, simulating photochemical reactions remains a computational challenge. Theoretical methods for describing excited states can be broadly divided into single-reference response methods and multireference methods. Single reference methods provide reliable semiquantitative results for single excitations. However, these methods cannot describe double-excited states, systems with strongly correlated ground states, or regions of degeneracy on the potential energy surface. The alternative, multireference methods, can provide more accurate results. However, multireference methods require significant technical and chemical insight and become computationally costly as the system size increases. I will discuss my work applying newly developed and well-known methods for understanding multistate processes. I will highlight the limitations and extent of current methodologies that prevent researchers from studying larger and more complex systems. I will also discuss new methodological developments using spin projection, which seeks to overcome several problems of single reference excited state models. I will illustrate the motivation and its performance compared to more established theories. Despite its success, the new method cannot account for ‘multiple correlation mechanisms’. As a result, I will introduce how multiple correlation mechanisms can be exploited to perform nonorthogonal active space decomposition, along with applications and paths for future improvements