Ions. Systems containing ions tend to exhibit much larger interaction energies as compared to cases where the monomers are charge-neutral, and this may place different demands on basis sets. We next consider results for the IL16, AHB21, and CHB6 data sets of ion-containing dimers that were described in Section 3 .4. Despite some promising preliminary results using XSAPT + MBD to describe ionmolecule interactions, we have identified other cases where performance of the MBD model is erratic for ion-containing systems, perhaps due to issues relating to the non-uniqueness of the Hirshfeld partition in such cases, for which iterative schemes have been suggested and implemented with supramolecular DFT-based versions of the MBD model. (Alternative means of obtaining polarizabilties for use in the MBD model have also been shown to be superior to the original MBD@rsSCS approach. ) In view of these difficulties, we consider that a proper MBD model for ion-containing systems is still under development. For the present purposes, however, we have already demonstrated that the MBD dispersion energy converges rapidly with basis set, because the density converges rapidly, and do not expect this to be different for an ionic MBD model. We will therefore focus on the other energy components in this section, using the XSAPT + ai D3 method. In this approach, E disp is modeled using atom-atom C 6 /R 6 and C 8 /R 8 potentials and does not depend on the density at all, but the other three energy components are computed in the same manner as in XSAPT + MBD, so the conclusions should be transferable.