In the case of 3I40, the error is reduced by a factor of six, and in the case of 1CTF and 2AK3 by a factor of four. Note that while the absolute errors are still rather large for the largest proteins, the errors are below 5 kJ/mol per amino acid fragment in all cases. The resulting error in the total energies compared to a supermolecular MP2 calculation are shown in Fig. . In all cases, the error is drastically reduced with the MFCC-MBE(2) method compared to standard MFCC. For the β-sheet structure, which does not feature intramolecular hydrogen bonds, already MFCC is rather accurate and MFCC-MBE(2) is in almost perfect agreement with the supermolecular result. For the β-turn structure, which contains intramolecular hydrogen bonds between the two strands, MFCC produces a substantial error in the total energy, but almost perfect agreement is achieved with MFCC-MBE . MFCC results in the largest errors for the α-helix and the 3 10 -helix.