Generic electrostatic model of calcium/sodium ion channel [11]. The model describes the channel's selectivity filter as an axisymmetric, water-filled pore of radius R = 0.3 nm and length L = 1.6 nm through a protein hub embedded in the cellular membrane. A centrally placed, uniform, rigid ring of negative charge Q_f is embedded in the wall to represent the charged residues of real Ca²⁺/Na⁺ channels. We take both the water and the protein to be homogeneous continua describable by relative permittivities and ε_p = 2, respectively, together with an implicit model of ion hydration whose validity is discussed elsewhere. The moving monovalent Na⁺ and divalent Ca²⁺ ions are assumed to obey self-consistently both Poisson's electrostatic equation and the Langevin equation of motion.