Finally, we consider how the vibrational spectrum of water is modified by the presence of the ions and electrode. Our goal is to identify the subpopulation of OH bonds that produces the peak around 3400 cm -1 in the experimental VSFG spectrum and why this peak emerges only under an applied potential. We begin this analysis by computing the spectrum of all OH bonds in each of the simulations. We find that under all conditions, the total water spectrum was nearly indistinguishable from that of the pure liquid, since only a very small number of water molecules experience a local environment that differs significantly from the pure liquid. In particular, no spectral feature was observed near 3400 cm -1 . In Fig. .A, we consider the spectrum of only the water molecules within one Debye length (8 Å) of the anode. These interfacial water spectra do not differ signifi-cantly from the bulk, and lack any feeature in the 3400 cm -1 region. Repeating this analysis for only the first water layer, within 3 Å of the interface, also produced spectra that did not differ substantially from the bulk. The experimental signal therefore can not be attributed simply to water near the interface.