This index was designed to measure the electrostatic interactions between a particular residue and the rest of the protein.
Where qx are the electronic charge indices of each residue, rx represents the distances between the Ca of each residue and the center of the protein, rij means the distances between the Ca of each residue (residues with interaction distance lower than 4Å were not considered due to such local interaction are considered to be the same before and after the folding process) and is the averaged radius of the protein. See Table of index's parameters for value of kel.