where Scontact, Voverlap, and Eelec are contact surface, overlapped volume, and electrostatic interaction energy between the rotamer and other parts of the protein, respectively; f is the observed frequency of the rotamer given a backbone conformation; and Nphil is the number of totally buried nonhydrogen-bonded hydrophilic atoms at the interface. The values in the equation are optimized weights of the energy terms (the weight for Scontact was set to −1).