The nonbonded term consists of electrostatic interactions, which are fully detailed further below, and van der Waals interactions, which are treated using the same force field that is used for the MM subsystem. The bonded QM-MM interactions are only present when there are covalent bonds that cross the QM-MM interface. In such cases, the undercoordinated QM atoms are saturated by adding boundary atoms that are characterized by monovalent pseudopotentials and contribute a single electron to the QM subsystem. The only bonded terms that are included are those not described through the QM subsystem. Consider a string of bonded atoms that cross the QM-MM interface, i.e.