Figure compares surface electrostatic potentials for a protonated His(+)-Ile-His-Ile tetrapeptide, computed using either a conventional PCM with ε s = 78.4, or else HetPCM with ε solv = 78.4 and ε nonp = 4. The first histidine in the sequence is protonated and positively charged, so we select ε solv for the atoms of this cationic histidine and ε nonp for the other three residues.