Selectivity in reduced models in which the optimal geometry is adapted to best-fit K⁺ or Na⁺ (a binding site is K⁺ selective when ΔΔG_Na,K is positive and Na⁺ selective when it is negative). The curves with solid lines correspond to realistic models of KcsA and LeuT, and the curves with dashed lines correspond to putative KcsA and LeuT sites stabilizing an optimal geometry adapted by energy minimization to best-fit Na⁺ or K⁺, respectively. Except for the site Na2 of LeuT, the onset of selectivity is established only for very large values of the architectural rigidity λ_g, indicating that a fairly stiff geometry is required to counterbalance the inherent trend that is already observed in the confined microdroplet limit (λ_g = 0) with loosely restricted ligands. This is adapted from Yu et al. (2010b).