The general procedure is as follows. A finite cluster is generated from a crystal structure (or a guess thereof). The center of mass of the "central molecule" is chosen to be the cluster origin and all molecules that are at least partially within a cutoff radius from the origin are included. Cluster atoms are represented with an atom centered point charge to capture electrostatic interactions and a Lennard-Jones (LJ) pair potential to describe non-bonded interactions. Point charges are derived from a QM calculation in the surrounding point charge field and assigned to all equivalent cluster atoms. Background charges are then fitted to reproduce the exact periodic electrostatics at the QM atom positions and placed above the cluster surface.Charges are recalculated until self-consistency is achieved. For non-bonding interactions, LJ parameter from a forcefield are assigned to all cluster atoms. The latter parameters remain unchanged during the calculation, although they could also be recomputed together with the charges. This is not an inherent limitation of the method.