First, the QM region size was radially increased around the substrate in 0.5 Å steps to later compare this distance-based approach with the systematic PCVA approach. The composition of each QM region can be found in Table . The convergence of the DHAP VDD charge and of the reaction energy with increasing QM region size is shown in Fig. and, respectively. The DHAP charge drops significantly from about 0 to -0.28 with region 4 and then again slightly increases for regions 6 and 7, but in the end again converges to about -0.28. The reaction energy converges starting with region 3 to around 9 kcal/mol, which is in very good agreement with the results of Bash et al. of about 8 kcal/mol .