Geometry of the model. (A) Axial cross section through the central region of the cylindrical simulation cell containing the pore. The selectivity filter is the central pore region (R = 3.5 Å, L = 10 Å) confining the oxygen ions of the EEEE locus. The dielectric coefficients are ε_w = 80 (aqueous regions, light gray) and ε_p = 10 (protein and membrane regions, white and dark gray). (B and C) Spatial views of the surface grid used in discretizing the dielectric interface at the pore wall, and a snapshot of the eight structural oxygen anions (red) and two cations dwelling in the pore (green, Ca²⁺; blue, Na⁺). Ions are represented as charged hard spheres with crystal radii (in Å: Ca²⁺ 0.99; Ba²⁺ 1.35; Li⁺ 0.6; Na⁺ 0.95; K⁺ 1.33; Cl⁻ 1.81; O^1/2− 1.4).