Supersystem-corrected MBE(2) + δ frag interaction energies for 1MMQ are quite close to those obtained using uncorrected MBE(3), but at a fraction of the cost. Results for the other systems do not align quite as well. The average difference between MBE(3) and MBE(2) + δ frag estimates of ∆E int is ≈ 20 kcal/mol across all basis sets, although the difference increases marginally with increasing basis set size. The supersystem correction actually increases the disparity between MBE(2) and MBE(3) in several cases. In these cases, it seems that the two-layer approach is no substitute for MBE(3), at least when the supersystem correction is performed using the minimalbasis HF-3c model.