Spectra of the distribution of the energy of association of different serie of decoys. Native and decoy energy distributions for three complexes: 1AR0 (top), 1BJF (middle) and 1FBT (bottom), using the energy functions with two (left), six (middle), and 20 amino acid classes (right). The native energy is shown as a thin bar. Thick curves in the lefthand panels correspond to a random energy model for decoy energies; see main text. Our decoy energies are significantly more diverse than the random energy model.