While CPCM could be useful for improving SCF convergence of large molecules, it also impacts the electronic structure. We selected four representative zwitterions (glycine, β-alanine, taurine, and glycocyamine) and computed their Hirshfeld partial charge on oppositely charged groups. For these zwitterions, the negatively charged terminal is the carboxyl anion (or sulfonic anion for taurine), while the positively charged terminal is the amino cation (or guanidino cation for glycocyamine). It is known that GGA functionals suffer from density delocalization errors (DDE), which is reflected as the underestimated partial charges of gas-phase BLYP calculations compared to the CCSD(T) reference for the four zwitterions (Supplementary Material Figure ). Although adding the Hartree-Fock exchange can mitigate DDE, the needed percentage varies for different molecules and is often larger than the optimal percentage for accurate energy, e.g., 20% for B3LYP. Hence, the partial charges by gas-phase B3LYP calculations are also underestimated (Figure ). As CPCM is applied with increasing dielectric constant, ε, the magnitude of partial charges on both terminals monotonically increases, agreeing with CPCM's density localization effects as reported in the literature, as well as our analysis in Section II-A. For all four zwitterions, the cationic N-terminals and anionic C-terminals can get their B3LYP calculated partial charges recovered to the CCSD(T) reference values by an ε of about 1.2. The corresponding BLYP calculations need a slightly higher ε of 1.5 due to the lack of Hartree-Fock exchange mitigation (Supplementary Material Figure ). It is worth noting that these optimal ε values to bring DFT calculated partial charges into agreement with CCSD(T) calculated partial charges are very small compared to even the most nonpolar solvents, e.g., ε=2.0 for cyclohexane at 298.15K. To further verify CPCM's density localization effects, we repeated the partial charge analysis for B3LYP and BLYP with another four charge models (see Section III D). All charge models agree on the trend, and the optimal ε for reproducing the CCSD(T) results is always 1.2 and 1.5