The first two terms represent MBE(n) calculations at either the target (high) level of theory or else the affordable (low) level of theory. The final term (E super low ) is the supersystem energy evaluated at the low level of theory with no fragmentation, thus δ frag can be viewed as a low-level correction for the effects of fragmentation. In previous work on enzyme thermochemistry, the Hartree-Fock (HF)/6-31G method (sans polarization functions) was shown to be an adequate choice for E super low . Use of the 6-31G basis set keeps the cost relatively low as compared to other double-ζ basis sets, especially if the electronic structure program can take advantage of compound sp shells used in Pople basis sets. Due to the size of the enzyme models considered here, however, we will use HF-3c for the low level of theory in eq. 7; see Section 3 .2 .3.