In the previous section, we calculated the projected electric fields between two pore waters located at stable positions: one near the selectivity filter, one near the intracellular gate. To check the sensitivity of our approach to the nature and location of the electric field probes, we calculate here the electric field at the (moving) location of pore Na + s (see Methods). We give in Table We see that the three residues from the selectivity filter, Asp-361, Glu-927 and Lys-1406, and those from the carboxylate outer-ring, Glu-364 and Glu-930, are again top contributors. This shows that these residues govern the electrostatic potential in the pore, regardless of the location of the probes. However, we also observe some discrepancies between the selection in Tables and for Thr-359, Cys-925, Thr-1404 and Asp-1701, it is not specific to those. Therefore, it is more likely that these residues exert a short-range force on Na + passing through the selectivity filter, that is undetected when computing the field across the central cavity.