In what follows, we consider errors in DFT-based MBE(n) calculations, meaning that eqn. (1) is truncated at n-body interactions. Explicit expressions for the n-body corrections (∆E IJ , ∆E IJK , etc.) can be found elsewhere. Calculations were performed with the Fragme∩t code interfaced to Q-Chem, as described in Section 5. For calculations on ion-water clusters X ± (H 2 O) N , the property of interest is the ion's interaction energy with the water cluster, ∆E int . Errors in MBE(n) approximations are defined with respect to a counterpoise (CP) corrected, 86 supramolecular calculation of ∆E int at the same level of theory that is used for the n-body calculations. The CP correction in the supersystem benchmark is useful for comparison to CPcorrected MBE(n) results but it amounts to a constant offset and does not affect the oscillations that are the primary focus of this work.