Although the increased HOMO-LUMO gap caused by CPCM benefits the SCF convergence, we need to know whether this increased gap is physically meaningful. For example, some hardto-converge molecules may be intrinsically gapless and have significant multireference character, making DFT the inherently wrong tool to study these systems, even with CPCM's help to converge SCF. We performed two additional tests to examine whether our zwitterionic molecules fall into this situation. For four small zwitterions (glycine, β-alanine, taurine, and glycocyamine) with small HOMO-LUMO gaps at the DFT level of theory, we estimated their accurate HOMO-LUMO gaps by calculating the vertical ionization potential (IP) and electron affinity (EA) with CCSD(T). We found that IP CCSD(T) -EA CCSD(T) values are much larger than DFT calculated HOMO-LUMO gaps, indicating that the molecules are not intrinsically gapless (Supplementary Material Figure ).