Interaction with light plays an important role in many biological processes. For instance in vision and photosynthesis photon absorption initiates a cascade of events, which leads to the biological response. Mimicking these processes in artificial systems could be a promising paradigm for construction of molecular devices. Understanding how the initial dynamics control the outcome is therefore essential. As the relevant time and spatial resolution can be difficult to access experimentally, we make use of computer simulations within QM/MM framework to gain atomistic insights into these processes. I will present the results of our simulations in various photoactive biological systems (Dronpa, PYP, Phytochrome), as well as their experimental validation by means of femto-second X-ray crystallography and spectroscopy.