Description of excited states in photochemistry with theoretical methods

The theoretical treatment of molecules in electronically excited states is much more complicated than in the ground state (GS) and remains a challenge. In contrast to the GS, electronically excited states can hardly be treated by a single determinant or configuration state function, not even near equilibrium geometry. This calls for multireference methods, or, alternatively, for time-dependent response methods, such as time-dependent density functional theory, or time-dependent coupled cluster response theory. In this contribution, we provide an overview on the latter techniques and illustrate on several examples how these methods can be used to theoretically investigate photoreactions.