This index counts the number of backbone H-bonds for each residue.
Where takes value one if the nitrogen atom of residue i is H-bonded with the oxygen atom of residue j and cero otherwise, in the same way
takes value one if the oxygen atom of residue i is H-bonded with the nitrogen atom of residue j and cero otherwise. The conditions that are considered to classify an O---N pair as H-bonded are summarized in the following figure: