Although we have assigned the water subpopulation responsible for the experimental peak, this population is present in similar quantities across all simulation conditions, including bulk water and near a neutral electrode. It remains to show why this peak only emerges in VSFG under an applied potential. Sum frequency generation is a surface-sensitive technique that is sensitive only to regions where inversion symmetry is broken. Therefore, if the orientational distribution of a collection of OH bonds contributing to a spectral feature is inversion symmetric, that feature will not appear in the VSFG spectrum. Noah-Vanhoucke et al have shown that this breaking of inversion symmetry in water is well-described by considering only the projection of the OH bonds along the z direction orthogonal to the interface. In Fig. .D, we consider this orientational distribution for different simulation conditions. In the pure water simulation (black trace), the OH bond distribution is isotropic, indicating that bulk water will not contribute appreciably to the VSFG spectrum -even if it has a sub-population of first solvation shell waters in the spectral range of interest. The orientation distribution near a neutral electrode(red traces), while anisotropic is still inversion symmetric and thus will also not contribute a VSFG signal. When an external field is applied, water molecules near the interface will align in response to the applied field, breaking inversion symmetry and therefore contributing to the VSFG spectrum. The role of the external field is therefore to align the waters in carbonate's solvation shell, rendering them VSFG active and producing the observed experimental signal.