In the T4 lysozymes, errors associated with fragmentation appeared to stabilize as the size of the model system increased (Fig. ). Here, we perform analogous testing using 1O48 where the ligand is much larger. Figure plots the results for a sequence of radial models of increasing size, comparing MBE(2) and MBE(3) calculations to unfragmented (supramolecular) values of ∆E int computed at the same level of theory, namely, DFT in either a double-or a triple-ζ basis set. The largest model in Fig. Å radius and contains 619 atoms but convergence to within 1 kcal/mol is achieved using a 3 Å model with 381 atoms.