In principle, highly accurate MD trajectories can be obtained from electronic structure methods like density functional theory (DFT). Unfortunately, such ab initio MD (AIMD) simulations require the (approximate) solution of the electronic Schrödinger equation at every time step. This makes them very expensive from a computational perspective and ultimately limits the applicability of AIMD to a few hundreds of atoms and relatively short (i.e ps) timescales. For many scientific questions, simulations of much larger systems, longer timescales or (usually) both are required. To this end, empirical interatomic potentials are typically used. These provide an analytical expression for high-dimensional potential energy surfaces which can be evaluated in a small fraction of the time required for a DFT calculation. This gain in efficiency invariably comes at the expense of a decrease in accuracy and/or transferability, however.