Figure 4.

Density plot of the joint probability distribution, ${\text{P}}_{\text{x}}(\epsilon ,{\text{U}}_{\text{CO-CO}}).$ ε is the binding energy between the ion and the carbonyl groups comprising the S₂ site. U_CO−CO is the excess internal energy of the S₂ site (but without considering the ion), which results from the repulsive interactions within the binding site ligands. The numbers indicate the ion coordination number for λ = 2.7 Å for Na⁺ and λ = 3.1 Å for K⁺. Observe that the low coordination states contribute to the high ε part of the distribution and vice versa. The region corresponding to ±40% probability around the mean is indicated. $\Delta \langle \epsilon \rangle =-21.0$ kcal/mol and $\Delta \langle {\text{U}}_{\text{CO-CO}}\rangle =7.3$ kcal/mol. Figure adapted from Dixit et al. (2009) with permission from Elsevier.