In contrast, PBE-based MBE(n) calculations are subject to wild oscillations that grow worse as n increases; the expansion appears to be divergent for this and other semilocal functionals that we have tested. Histograms of the various interaction terms ∆E IJ••• are plotted in Fig. for the HF/aQZ and PBE/aQZ calculations, and summary statistics are provided in Table . The histograms are separated into fluoride-containing subsystems, which afford larger interactions on average, and those that contain only water molecules. Even for the seemingly divergent PBE-based expansions, the magnitude of the n-body corrections does decrease with n. However, for the PBE calculations the net contribution from the fluoride-containing subsystems increases as a function of n, leading to the observed divergence. For PBE, the total contribution from the fluoride-containing terms is -115.9 kcal/mol for n = 4 and +193.0 kcal/mol for n = 5. These values are unmatched in the wateronly terms, which sum to -4.0 kcal/mol (n = 4) and +1.8 kcal/mol (n = 5), although these values are still significantly larger than the water-only terms obtained at the HF level, which sum to -0.6 kcal/mol (n = 4) and +0.1 kcal/mol (n = 5). Thus, the divergent behavior is exacerbated by the presence of an anion. A combinatorial increase in the number of n-body terms results in divergence for PBEbased MBE(n) despite the fact that individual ∆E IJ••• corrections decrease order-by-order. Our data are consistent with previous studies that observed a marked increase in errors for MBE-based energy decomposition analyses (based on two-body terms only) when GGAs were employed. Those studies, however, were limited to (H 2 O) 6 clusters and these do not engender the rapid divergence that we observe using F -(H 2 O) 15 .