Calculate the solvation free energy

GROMACS version:
GROMACS modification: Yes/No
I calculated the solvation free energy using the gmx sasa code, and I know that this code estimates these energies from per-atom solvation energies per exposed surface area. Could you explain how the gmx sasa code calculates these energies, please? Does Gromex use a specific formula to calculate this energy?

gmx sasa with the option -odg calculates solvation free energy based on the following article (in the article you find the formula used)
D. Eisenberg and A. D. McLachlan
Solvation energy in protein folding and binding
Nature 319 (1986) pp. 199-203