As the results for 16-residue QM regions do not follow a clear trend, we added the 17thranked residue to the QM region, which is the catalytic HIS95 residue in both cases (labelled standard+95 and cryst+95 ). This residue is also detected by CDA and is thus potentially relevant. Unfortunately, the inclusion of HIS95 increases the DHAP charges for standard and cryst significantly. Interestingly, the reaction energy for standard PCVA is not affected, while for cryst the energy is improved to about 7 kcal/mol, close to the best estimate. Overall, these results underline the ability of single amino acids to strongly change QM charges and reaction energies. This indicates the need to further develop new approaches based on PCVA which do not only account for electrostatic effects and are able to better detect catalytic residues. Additionally, with respect to the 14-residue QM region 5' being an outlier in DHAP charge and reaction energy (see Fig. ), an atomeconomical QM region size of around 16 amino acids seems not to be adequate for the TIM system to get reliable and consistent results. Thus, the use of a predefined, fixed size for an atom-economical QM region has to be revisited in future work.