For exchanges of large ions to small ions, as in the case of K + to Na + , K + to Li + , and Na + to Li + , the most important feature from feature selection is either the average Hirshfeld charge or the average local ionization energy of the coordinating atoms. Whereas for the exchange of small ions to large ions, such as Na + to K + (except for the ridge model) and Li + to Na + (except for the KRR model), the most important feature is the average relative ion-coordinating atom distance. The model for the Li + to K + exchange displayed low accuracy (>5 kcal/mol MAE and <0.5 R 2 for some models) and hence no physical meaning was inferred from feature selection. The trend of the top- ranked features suggests that when a complex changes from a larger to smaller ion, the most important features to determine the favorability of this change are the average local ionization energy and Hirshfeld charge, both measures of the electronic structure. On the other hand, for complexes where a small ion is replaced with a larger ion, the distance between the nuclei of the ion and coordination atoms are more dominant in controlling binding affinity. In such cases, the introduction of a large ion into a smaller cage creates repulsive forces, resulting in strain that depends on the flexibility of the complex.