Physico- and bio-chemical processes on the femto- to picosecond time scale are ideally suited to be investigated with all-atom simulations. They include, amongst others, vibrational relaxation, ligand migration in sterically demanding environments (proteins, ices), or vibrational spectra. By comparing with experimental data, the results can be used to obtain an understanding of the mechanisms underlying the observations. Furthermore, most of these processes are sensitive to the intermolecular interactions. Therefore, detailed refinement of such interaction potentials is possible.