In contrast to the distance-based case, a slightly increasing trend is observed for the reaction energy sensitivity (Fig. ). For QM region 8', at which the reaction energy is further from our best estimate, the sensitivity also shows a marked increase. Except for the smallest QM regions, the global PCVA sensitivity can thus be used to judge the accuracy of the calculated QM/MM reaction energy. The overall larger sensitivity for the PCVAbased QM regions compared to the distance-based construction could be an effect of a larger contact area between the MM and QM regions because in the PCVA case, MM residues close to the active site with low sensitivities remain in the MM part and can affect adjacent QM residues. (e.g., VAL41, GLU89, or ASP140). Moreover, also more distant residues are included that would not be considered in a 16-residue QM region in the distance-based case (e.g., MET39, LYS45, ALA66, or ASP168). This indicates the ability of PCVA to even detect high-impact residues located relatively distant from the active site COM. Nevertheless, there are several residues included by CSA and/or FSA that are not under the 16 highest-ranked residues when applying the PCVA approach. However, most of these residues are assigned a PCVA rank close to 16 such as ASN40, GLU63, or ILE90.