Stabilization of 2D-lattice model proteins by taking into consideration the contact frequency of residue pairs in contact and not in contact in the native state. The average free energy of folding of 100 sequences designed either with (○) or without (△) taking into account the contact frequency is plotted against the value of the contact weight, Wc (see Eqs. (8) and (9), used in the design. The results shown here are for sequences with L = 30 on a 5 × 6 lattice. Similar results were obtained in cases of other lattice dimensions and sequence lengths when only the most compact conformations were considered. For more details, see Methods.