Theoretical Methods

The description of the electronic structure of molecules in the excited state is usually more involved than the calculation of ground-state properties. The development of approaches to calculate optical properties of chromophores, including their specific interactions with a complex environment is a very active field of research. This chapter gives an overview of quantum mechanical methods and schemes to integrate them into a multi-scale description of extended systems that contain an optically active center. Special attention is paid to the problems and limitations of quantum methods that are commonly used to describe excited-state properties of biological chromophores.