Simplified calculation of folding energies and residue coordination numbers in random heteropolymers

I develop a formalism for calculating effective pair and higher-order interactions between residues in random heteropolymers that approximately predict the folding enthalpy and the coordination numbers of individual residues. In a simple model heteropolymer with additive couplings between residues, the folding enthalpy is written in terms of two-, three-, and four-body interactions between residues. The coordination numbers are expressed in terms of interactions between up to three residues. Application to a 6x6 square model shows that the folding enthalpy is obtained to an accuracy of better than 1%. The coordination numbers are obtained with a rms error of 1.2 neighbors.