At this time, one can speculate about some of the reasons leading to the systematic overbinding observed in our AC(2) scheme. So far, the 2-body contributions are being added according to distance criteria. Preliminary tests run for the smaller systems show convergence in the binding energy up to 1.5 kJ•mol -1 with the distance selection of ca. 3.5 Å (7 a.u.). In the case of ammonia, the criterion was enlarged to 9 a.u. to ensure the energy convergence. The geometries were found to converge even earlier. These tests have been repeated for cases where the deviations were above 10 kJ•mol -1 , but this only leads to marginal improvements. Another systematic error is expected to stem from the QM/MM description of higher order effects. While electrostatics are captured from the point charges present during the twobody calculations, exchange contributions are not. Exchange repulsion is included into the QM/MM energy through the LJ potentials, but it does not influence the QM wave function. For this reason it seems reasonable that the systematic lack of many-body exchange effects could contribute to an overbinding effect.