We give in Supporting Information the list of the top contributors in each case. We observe that the main contributors to the field along the S4 helix of a given domain are mostly residues from that domain. We find that many top contributors are located along the respective S4 helices, but we also observe contributions from residues in S2 and S3. This is consistent with other studies that reported that residues in S2 and S3 correlate with the gating charge residues to facilitate the upward motion of S4. Additionally, Leu-834 and Gln-886, both identified as key contributors to the fields along S4 (17.91 and 4.50 MV/cm, respectively), are known pathological mutation sites yielding paroxysmal pain syndrome, and primary erythermalgia, a neural disorder caused by hyperactive Na v 1.7. This suggests that electric field calculations could enable the identification of key residues as targets of therapeutic intervention, similarly to recent electric-field based enzyme design approaches.