Regarding VDD charges (see Fig. ), PCVA performs as well as CSA and FSA. The change in the Mg 2+ charge to about 0.3 with the distance-based region 8 (34 residues) and larger is achieved already in the QM regions containing 16 residues. For SAM, all three atom-economical QM regions also provide the converged charge. In the SAM case, none of the methods is able to detect the charge change to 0.25 for region 7 and larger. This is a fundamental limitation of QM regions of a given size and can probably only be rectified by using larger QM regions.