The trouble starts when applying the fragment-fragment two-body contribution as given by Eq. ( ). Here, the interaction between the blue and the red peptide group is included four times instead of only once. It appears in the calculations for the fragment-fragment dimers A-D, A-E, B-D, and B-E (see top right part of Fig. ). Because the ACE and NME groups that are added as caps can form hydrogen bonds to other amino acid fragments and their caps, these hydrogen bonding interactions are counted multiple times ) method for a short fragment of a β-sheet. The hydrogen bonding interaction between the peptide groups highlighted in blue and red appears in four fragment-fragment interaction terms, in four fragment-cap interaction terms, and in one cap-cap interaction term. See text for details.