When atom-centered Gaussian basis sets are used, this approach suffers from the well-known deficiency of basisset superposition error (BSSE). This reflects that fact that in the dimer calculation (E AB ), either monomer may use basis functions centered on the other to improve the quality of its wave function, but that possibility is precluded in the monomer calculations (E A and E B ). This leads to significant overestimation of ∆E int that disappears slowly as the monomer-centered basis sets approach completeness. The Boys-Bernardi "function counterpoise" (CP) procedure is a means to sidestep this problem, which dates to the early days of quantum chemistry. The CP procedure recognizes that eq. 1 is