With the incorporation of four-body terms that metric achieves the strict criterion of 0.1 kcal/mol/monomer, but at the same time the number of subsystem calculations becomes nearly intractable for any level of theory beyond semi-empirical calculations. Large three-body terms in 1ZP5 cause require higher-order expansions but in all of the remaining systems, the error per monomer is below the target accuracy already at the MBE(2) level. In other fragment-based studies of metalloproteins, the fragment that includes the metal ion cofactor typically contains all proximal molecules (side chains and crystallographic water molecules). However, this is not possible in the case of the metalloproteins investigated here because the ligand coordinates to the metal ion, and since the ligand must be removed in order to compute ∆E int , no atoms from the enzyme can be included in the ligand fragment(s).