∆φ orient , the work function reduction due to the net dipole orientation of water molecules, was calculated by isolating the solvent (H 2 O) molecules and calculating the outer (Volta) potential difference between the vacuum regions on either side of them . We see in Figure that φ vac DFT systematically differ from experimental values by about -0.36 eV, which is consistent with previous studies using the RPBE functional without dispersion corrections . As shown in Figure , the computed and experimental values of ∆φ show a similar deviation of 0.41 eV, but with the opposite sign. This shift -being constant for all metal substrates -may arise either from the water-vacuum interface, present in all our simulation cells or alternatively by an RPBE functional dependent underprediction of water binding (e.g. in comparison to PBE-D3 ) which may alter U PZC (see below). The two errors shown in Figures and in a larger magnitude of ∆φ . Interestingly, the 0.24 eV work function reduction seen on weak-binding surfaces of Ag, Au and Cu is similar to the work function reduction ∆φ calculated using an implicit solvent model like VASPsol (see Table in the SI).