However, as these 1-3 interactions cannot be expected to be negligible, we decided to calculate these terms indirectly as illustrated in Fig. . Note that this requires calculations for trimers of directly connected amino acids. However, we still consider MFCC-MBE(2) a two-body expansion, as these trimer calculations are actually used to calculate specific (two-body) fragment-fragment interaction energies indirectly. Moreover, the number of considered trimers scales linearly with the number of fragments and thus does not increase the scaling of the computational effort with system size.