To gain a more complete understanding of the origins of selectivity in ion binding, we drew inspiration from the activation-strain model and decomposed DEtotal to an interaction-based component, DEint, and strain/reorganization-based component, DEstrain. The interaction-based component represents only the binding energy change one ion experiences when moving from the molecular complex to water and vice versa. The strain component represents the geometric rearrangement energy necessary for the complex to adjust to the new ion. We find that interactionbased energy contributes the majority of DEtotal for most of the ion exchanges studied (Figure ).