Computational Photochemistry

This chapter discusses photochemical reaction path concept and its use in mechanistic investigations. The field of computational photochemistry is a relatively young field, especially when applied to the study of ultrafast reactions, but it is now established as a branch of computational chemistry and as a powerful, sometimes unique, way to simulate the molecular mechanism underlying fundamental chemical and biological events such as vision, primitive photosynthesis, phototropism, photochromism, bleaching, fluorescence, phosphorescence. These days, computational strategies are available for locating conical intersection and singlet/triplet crossing points and for constructing inter-state “photochemical” reaction pathways. These tools comprise methodologies for the optimisation of low-lying crossings between pair of potential energy surfaces and the computation of relaxation paths from a photoexcited reactant (For example, from theFranck-Condon (FC) structure) to a deactivation channel.