Together, these results demonstrate that the application of MBE(n) with vacuum boundaries, to an enzymesubstrate model extracted from a crystal structure, need not converge to the supermolecular result at low orders, n ≤ 4. This behavior results from charge (de)localization that may vary greatly from monomer to dimer to trimer, etc., when the subsystems contain fragments with net charge. Elsewhere, low-dielectric boundary conditions (ε ≈ 1.5) have been shown to reduce density delocalization error in isolated-peptide DFT calculations, and in the present context the use of ε = 4 appears to prevent oscillatory changes the corrections ∆E IJ , ∆E IJK , . . ..