We investigated the DFT convergence of 25 peptides in their zwitterionic and non-chargeseparated form and concluded that CPCM could only effectively reduce the SCF iteration steps for charge-separate systems. The reason is that CPCM can open the near vanishing HOMO-LUMO gap in the zwitterionic peptide and avoid the vigorous virtual-occupied space mixing over iterations, the origin of convergence issues. Non-charge-separated molecules do not suffer from the vanishing gap problem and, therefore, their convergence does not benefit from CPCM even though their HOMO-LUMO gaps are also slightly increased by CPCM.