This index follows the classical definition of the superficial free energy as the product of the surface tension and the area of the species.
Superficial tension is represented with the Kyte-Doolitle hydrophobicity scale (Kyte and Doolitle, 1982). The area is estimated as the average value of surface area for each type of residue in fully extended protein chains.
Kyte, J., and Doolitle, R.F., 1982. A Simple Method for Displaying the Hydropathic Character of a Protein. . J. Mol. Biol. 157, 105-132.