Before revision:  N_i( ) is the mean number of water molecules in the first hydration layer that directly contact to the atoms with the hydrophobicity degree i; \mu_i is the chemical potential. Label all atoms of hydrophobicity degree i by H_i and let M_\bX be the molecular surface for the conformation , defining M_{\bX_i as the set of points in M_\bX that are closer to atoms in H_i than any atoms in H_j, j } i. Then the geometric version of G( ) resembles to well-known surface area models , plus a volume term:
After revision:  N_i(  , defining M_{\bX_i as the set of points in M_\bX that are closer to atoms in H_i than any atoms in H_j, j } i. Then the geometric version of G( ) resembles to well-known surface area models , plus a volume term: