In the following subsections, we will investigate the applicable molecular systems and mechanisms for CPCM as an SCF convergence accelerator. We will first test the performance of CPCM in improving SCF convergence on zwitterionic and non-charge-separate peptides and amino acids. We will then compare CPCM with a conventional SCF convergence accelerator, level-shifting, and investigate the origin of their different performance despite their similar effects in opening the HOMO-LUMO gap. Finally, we will demonstrate the CPCM's reduction of DDE in the tested molecules while being used to accelerate SCF convergence. A. The Impact of CPCM on SCF Convergence. We compared the SCF convergence of the B3LYP/6-31+G* calculations for 25 zwitterionic peptides and their non-charge-separated counterparts in the gas phase and CPCM (Figure and Supplementary Material Table ). It is known that CPCM's convergence improvement effect is most prominent for pure GGA functionals 9 like BLYP and PBE. However, neither functional can generate converged gasphase calculations for any of our benchmark molecules, making it hard to compare the SCF convergence quantitatively. Therefore, we chose the B3LYP functional, which had converged SCF but in an undesirably large number of iterations. In the gas phase (e=1), the charge-separated peptides are always harder to converge than their non-charge-separated counterpart: all noncharge-separated peptides converge within 61 DIIS iterations while 17 out of 25 peptide zwitterions take over 100 iterations or do not converge within 1000 steps. Applying CPCM with even a low dielectric constant of ε=2 (corresponding to non-polar solvents like cyclohexane) significantly reduces the SCF iterations for all zwitterionic peptides by at least 63% (Figure ), with the only exception, 1aqg. When a high dielectric constant of ε=80 (corresponding to water) is applied, the DIIS iterations for 19 out of 25 peptides are reduced to <50% of the gas-phase results. In contrast, the SCF convergence of non-charge-separated peptides seems to benefit little from CPCM in the whole range of e values tested (2 to 80), with only 3 out of 25 non-chargeseparated peptides getting marginally reduced iterations. Our result demonstrates that CPCM can only effectively reduce the SCF iterations for zwitterionic peptides.