Every atom of a crystal can be generated from the asymmetric unit by a combination of symmetry operations and translations. If the asymmetric unit contains not more than one molecule (or a part thereof, in case of internal symmetry), one may use a single molecule to be the "natural" choice for the smallest unit to represent the periodic system. Every molecule of the cluster can be generated from that single molecule and, due to symmetry, it will have the same environment and thus in-teractions as any other molecule found in the crystal. All of the information about the crystal is contained therein. The AC-QM/MM model is based on the idea of a single QM molecule (in the following denoted as "central molecule" with the index 0) which is surrounded by a finite MM region. The environment does not interact with itself, thus all interactions take place either within the "central molecule" or between the "central molecule" and its environment. The periodicity of the system beyond the selected MM environment is described by a surrounding embedding charge field.