Ab initio investigation of photochemical reaction mechanisms: Fromisolated molecules to complex environments

This chapter focuses on the computational investigations of light-induced chemical reactions in different systems ranging from organic molecules in vacuo to chromophores in complex protein environments. The aim is to show how the methods of computational photochemistry can be used to attain a molecular-level understanding of the mechanisms of photochemical and photophysical transformations. Following a brief introduction to the field, themost frequently used quantum chemicalmethods formapping excited state potential energy surfaces and for studying the mechanism of photochemical reactions in isolated molecules are outlined. In the following sections, such methods and concepts are further developed to allow the investigation of photo-induced reactions in solution and in the protein environment.