  By exact computer enumeration and combinatorial methods, we have calculated
the designability of proteins in a simple lattice H-P model for the protein
folding problem.
  We show that if the strength of the non-additive part of the interaction
potential becomes larger than a critical value, the degree of designability of
structures will depend on the parameters of potential. We also show that the
existence of a unique ground state is highly sensitive to mutation in certain
sites.
