A. Peptides and Small Zwitterions. To test CPCM's SCF improvement effects for large molecules, we selected a set of 25 canonical zwitterionic peptides from the RCSB PDB database obtained with aqueous solution nuclear magnetic resonance (NMR) or X-ray diffraction. The peptides range from 6 to 26 in the number of residues and 64 to 440 in the number of atoms, and most were found to converge within fewer SCF iterations in CPCM and XP-PCM 25 than in the gas phase in previous studies. Detailed properties of the peptides (PDB ID, number of residues and atoms, charge, and secondary structure) are summarized in the Supplementary Material, Table .