3.2. Selecting τ 2B . We next consider the construction of enzyme models via the two-body energy criterion in eq. 6. Table compares errors for MBE(2) and MBE(3) approximations for models generated in this way. All calculations were performed at the HF-3c level and the error is defined with respect to a full-system calculation performed at the same level. Timings for the full-system calculations can be found in Table .