To assess the accuracy of such a distance-based screening of the two-body interactions, Fig. plots the errors in the total energy calculated with MFCC-MBE(2) using hydrogen caps and ACE-NME caps as a function of the cutoff λ. Note that the errors shown for λ = ∞ correspond to those given in Table , whereas for λ = 0 all two-body interactions are neglected and the H-MFCC and MFCC methods are recovered. When using hydrogen caps, the cutoff can be decreased to λ = 4 Å before the error in the total energy starts to increase noticeably. With ACE-NME caps, it could even be decreased further to λ = 2 Å without affecting the accuracy significantly. This goes along with the length of a typical hydrogen bond in polypeptides of about 2-3 Å. . To be on the safe side, we selected a cutoff of λ = 4 Å for all further calculations.