While MFCC-MBE(2) drastically reduces the error in calculated total energies compared to the conventional MFCC method, the remaining errors in the total energies are still rather high in several cases. However, errors in total energies are not very meaningful for many applications and a much better accuracy can be expected to errors in relative energies due to systematic error cancelation. To assess the accuracy that can be reached for relative energies, we consider ten conform-ers of neurotensin complexed with the third extracellular loop of human neurotensin 1 receptor (PDB code: 2LYW ). This test case was previously established for quantumchemical fragmentation methods by Hellmers and König . 2LYW (675 atoms) contains two peptide chains with 13 and 24 amino acids, respectively. As in Ref. Our results are shown in Fig. , which plots the energies of the ten conformers relative to the lowest-energy one. The performance of the conventional MFCC method in reproducing the correct energy ordering of the conformers as well as the magnitude of the energy differences is very poor. For instance, conformers 5 and 3 are predicted to have almost the same energy by MFCC, whereas in supermolecular calculations their energies differ by ca. 200 kJ/mol. In contrast, MFCC-MBE( ) is able to reproduce the relative energies almost perfectly and predict the correct energetic ordering between all ten conformers.