is a two-body correction, etc. If eq. 1 is truncated at n-body interactions, then we call the resulting method MBE(n). In conjunction with high-quality basis sets and correlated wave function models, MBE(4) affords good accuracy for neat liquid water and monovalent ionwater interactions, yet requires electronic structure calculations on systems no larger than (H 2 O) 4 or X ± (H 2 O) 3 . However, four-body calculations manifest a crippling O(N 4 ) combinatorial prefactor, resulting in error accumulation for large systems. Replacing wave function methods with DFT reduces the cost but SIE becomes catastrophic, with fluctuations as large as ±200 kcal/mol in low-order MBE(n) calculations in clusters such as F -(H 2 O) 15 . This effect is only marginally reduced by a combination of standard hybrid functionals (such as PBE0 or B3LYP) and/or aggressive energy screening. For meta-GGA functionals including SCAN and ωB97X-V, these error mitigation strategies are only moderately effective. Because meta-GGAs are known to have more stringent grid requirements, we decided to revisit the quadrature grids used in the DFT-based MBE, even though previous MBE(4) calculations at the B3LYP/cc-pVDZ level suggested that the quality of the integration grid had a negligible effect on accuracy, in applications to (H 2 O) 40 . For water or ion-water clusters, MBE(n) should converge to same interaction energy (∆E int ) as a supramolecular calculation at the same level of theory, as n increases. (We use single-monomer fragments so that no covalent bonds are severed.) As such, it makes sense to define the error in the MBE(n) approximation as