The source of these errors remains a topic for future work and development, but we can speculate. It has been demonstrated that MBD is sometimes vulnerable to polarization catastrophes for short-range interactions, 149 as one might anticipate in strong interactions with ions, although the reason why cations are more problematic is unclear. Especially for ions, 173 the model is sensitive to the input polarizabilities, 149 which are controlled by the oscillator frequency ω a in Eq. (31). Alternative models for obtaining these polarizabil- ities have been suggested, 174 which are available in the libMBD library that is interfaced to Q-Chem, 175 but these have not yet been adapted for use with XSAPT. At a minimum, iterative determination of Hirshfeld volumes, to assign excess charge to appropriate atoms in a molecular density, is probably necessary to make MBD generally applicable to ions. However, it is unclear that this should improve the description of complexes involving monatomic ions, such as those in the CHB6 data set. Finally, a more rigorous treatment of dipole-quadrupole (and higher-order) polarizability contributions to MBD, as compared to the MBD + esDQ model in Eq. ( ), might improve the results for strong, short-range interactions.