Partition function zeros of the two-dimensional HP model for protein folding

The partition function of the two-dimensional lattice HP model for protein folding is computed by exact enumeration. For a protein-like sequence, the distribution of partiton function zeros shows roughly a two-ring pattern, while for a nonprotein-like sequence, the outer ring of zeros is ill-developed and cannot induce the folding transition. By tracing the peak or shoulder of the heat capacity in the complex plane, the phase boundaries can be determined along the positive real axis.