We are interested in processes which occur in or between molecules at the microscopic level. These processes may be photochemical, photophysical, physical or chemical processes in small organic molecules and proteins. To investigate these processes we perform computer simulations, which help to understand theses processes at an atomic scale. Which methods we apply depends on the kind of problem and on the size of the system. For studying processes in small methods we apply a wide range of quantum mechanical (QM) methods: semi-empirical methods, (time-dependent) density functional theory, post-Hartree-Fock methods such as MP2 and coupled cluster, and a combination of density functional theory and multireference configuration interaction (DFT/MRCI). Large molecules, such as proteins, are treated using Molecular Mechanics (MM) methods, for investigating chemical reactions often combined with some QM method (QM/MM). We also use chemotopological methods to improve our understanding of molecular behavior. In the following we describe some of our research.