While this does change the values of the sensitivities, it leads to overall similar trends (cf. Fig. ). Using the product or reactant energy sensitivities could also be advantageous for avoiding error cancelation that might be present when considering the sensitivity of the reaction energy only. Consequently, we choose to use only the reactant energies, which reduces the number of calculations to be performed by half. The impact for this approximation on the selection of the QM region will be discussed below. Finally, instead of using a symmetric two-point formula for the numerical differentiation, it turns out to be sufficient to use a forward finite-difference formula (PCVA-E), which again reduces the number of QM calculation by another factor of two.