For the protonated histidine residue, there is essentially no difference in the surface charge predicted by the isotropic PCM and HetPCM methods (Fig. ). However, subtle differences emerge on the other three residues, which are described with nonpolar boundaries in HetPCM but are solvent-exposed in the conventional PCM calculations. In the latter case, there is buildup of charge associated with the nitrogen and oxygen atoms that is not observed with HetPCM, where the residues in question are exposed to a low-dielectric environment that apparently cannot support such a charge buildup. We expect this to be important in biomolecular modeling, in order to prevent anomalous buildup of charge on what are supposed to be hydrophobic parts of the surface. In contrast, and not unexpectedly, the surface charge at the alkyl groups is more similar between these two models. Whereas tests against exact Poisson boundaries in Section 4 .3 suggest that HetPCM should probably not be used to predict absolute solvation energies, results below demonstrate that the use of an appropriate reference state does allow calculation of a relative solvation energy, and thus a pK a . This methodology builds upon work by others to compute relative ∆G • and pK a values using a heterogeneous PCM. We also provide an assessment of how the HetPCM results change with respect to the value of ε nonp that is used for the surface boundary of the hydrophobic portions of the protein model.