We also investigated why the exception, zwitterionic 1aqg, does not get SCF acceleration from CPCM. Compared to other peptide zwitterions, 1aqg has a significantly larger HOMO-LUMO gap of 4.049 eV and can converge easily in the gas phase within similar iterations as the non-chargeseparated counterpart. As shown in Figure , further investigation of this peptide's structure shows strong salt bridges and ionic hydrogen bonds between oppositely charged residues. All three carboxylate anions are within 2.2 Å of an amino cation, electrostatically stabilizing each other and making this peptide resemble a non-charge-separated molecule. In contrast, other peptide zwitterions have at least a pair of oppositely charged residues separated by more than 6.0 Å. B. Comparison to Level-Shifting. Although we have already observed the increased HOMO-LUMO gap and reduced SCF iterations in CPCM calculations, it is unclear whether CPCM's acceleration effects can be attributed solely to the increased gap. To deepen our understanding of the acceleration mechanism, we compared the performance of CPCM to level-shifting, a more conventional technique with a similar mechanism focusing on increasing the HOMO-LUMO gap.