The smallest of the T4-lysozyme systems contains 2,636 atoms, which taxes our ability to perform supersystem benchmarks using high-quality basis sets. In an effort to obtain more convergence data, we turn to HF-3c where supersystem calculations are more feasible. Results in Table demonstrate that MBE(2) estimates of ∆E int consistently fall within 1 kcal/mol of the conventional supramolecular result obtained at the same level ) cost somewhat, because the full-system calculations were performed on slightly newer hardware as described in Section 2 .4. It is common in fragment-based calculations to report errors on a per-monomer basis, recognizing that overall errors may be size-extensive. A target accuracy of 0.1 × (3/2)k B T , or ≈ 0.1 kcal/mol at T = 298 K, has been suggested. This threshold represents 10% of the available thermal energy per fragment, with the idea that fragmentation errors should be rendered negligible in comparison to thermal fluctuations in the energy. While it's not clear that this is the right target accuracy for biomolecular ∆E int calculations, our T4-lysozyme calculations do achieve this stringent criterion: the largest errors in Table are only 0.006 kcal/mol/monomer.