Summary of the main computational results for the entry of K⁺ and Na⁺ in the selectivity filter of the KcsA channel from the extracellular side (Z > 12 Å) in the presence of two other K⁺ ions in the pore in a K⁺/K⁺/K⁺ or a Na⁺/K⁺/K⁺ configuration. (A) The PMFs for the entry of K⁺ and Na⁺ in the selectivity filter (the thin red line is the original PMF for Na⁺ before it was shifted by 0.7 Å along Z to match the wells and barriers of the PMF for K⁺). (B) Coordination numbers for the ion entering the selectivity filter. The average numbers of water, carbonyl, and total oxygen atoms closer than 3.2 Å from an entering K⁺ or 2.8 Å for the entering Na⁺ are shown. The number of coordinating water molecules decreases as the ion moves from the bulk phase into the narrow selectivity filter for Z < 7 Å. However, the total coordination number displays only small variations: over the entire entry process, the average coordination number of K⁺ increases slightly from 6 to 7, whereas that of Na⁺ remains constant around 5.5. This is adapted from Egwolf and Roux (2010).