Even with all the approximations and optimizations of the last hundred years, first principles quantum chemical calculations are still very resource demanding, even for quite small systems. The upper limit for a geometry optimization is around a thousand atoms and a dynamics simulation is only possible for systems of a few hundred atoms, and even then only for very short time scales. This means that unless new approximations are introduced, many systems that are of interest will be far beyond the capabilities of today’s computers. A small piece of protein from the human body can consist of several thousand atoms, and even very small systems can increase by hundreds of atoms when a solvent is added. A popular way of dealing with this problem when simulating the motion of atoms is to use molecular mechanics (MM).