Comparing MP2 and BW2 reveals two key points: First, the BW2 expression is indeed regular, because the denominator is prevented from going to zero by the presence of E and E . This is a clear advantage over MP2. Second, the BW2 energy is not size-consistent, because E and E are global quantities. Considering a system of two non-interacting fragments A and B, we can see that fragment B affects E (A)the BW2 energy of fragment A -by introduction of E (B) and E (B) into E on the right hand side of Eq. 26. This is a fatal disadvantage relative to MP2 and is why BW2 has languished.