This index constitutes a Lennard-Jones representation to the weak interactions between a residue and the rest of the protein.
Where rij means the distances between the Ca of each residue (in order to save time, residues with interaction distances larger than 10Å were not considered due to the marginal value of the Van der Waals interactions beyond this limit). The value 3.965 represents the minimum distance between two non-adjacent Ca atoms. See Table of index's parameters for value of kLJ.