We also made the non-charge-separated form for each zwitterionic peptide to investigate how CPCM's SCF improvement effect depends on the molecule's charge distribution. The non-chargeseparated structures were generated by transferring one proton from the N-terminal to the Cterminal with the remaining atom coordinates fixed using Avogadro 1.2.0. To investigate the impacts of PCM on the electron density in detail, we selected four typical small zwitterions (glycine, β-Alanine, glycocyamine, and taurine) that can represent the charged residuals in peptides and are small enough to be calculated at the coupled-cluster level of theory. All the small zwitterion structures were downloaded from PubChem. 28 B. DFT Calculations. All DFT calculations in the gas phase and CPCM were carried out in the GPU accelerated quantum chemistry package TeraChem. The SCF convergence of peptides in the gas phase and CPCM were tested at the B3LYP 30 /6-31+G* level, where the global hybrid functional, B3LYP, has its DDE partially mitigated by HF exchange but still benefits from CPCM's effects. The larger basis set, aug-cc-pVDZ, was used when comparing the DFT electron density of small zwitterions with coupled-cluster reference results. The DIIS 36 algorithm was used for all SCF calculations and iterated until the largest component of the DIIS error vector was smaller than 1 × 10 )F Hartree. All electron integrals were evaluated on the GPUs using double precision arithmetic to isolate CPCM's impacts on SCF, avoiding potential influence by TeraChem's mixed precision or dynamic precision techniques. This also guarantees that conclusions in this work also apply to calculations done with CPU-based quantum chemistry packages where only double-precision arithmetic is available.