The variation of the population analysis and the LJ parameter definitions has a significant impact on the AC(1)-QM/MM results, since intermolecular interactions are solely represented at the QM/MM level. At the AC(2)-QM/MM level, however, results are essentially independent of this choice. Probing different combinations of NPA or Chelpg charges with LJ parameters from CGFF and MM3 force fields for a subset of the X23 set results in cohesive energies that vary by at most 2 kJ•mol -1 , see Figure . The major difference observed was in the stability of the optimisation procedure, with Chelpg charges leading to a slower convergence pattern. We have opted for the combination of NPA charges and CGFF parameters. The latter exhibit the most robust behaviour among the different choices.