In order to determine the Poisson potential of bulk electrolyte systems, we have explored the finite-field MD simulations of bulk electrolyte solution to speed up the statistical convergence. Because electrolyte solutions are ionic conductors, the Maxwell electric field E vanishes on statistical average and the electric displacement D is related to the supercell polarization P by the relation D = 4π⟨P⟩. This allows us to apply classical MD simulations at D z = 0 with the MetalWall code to the same bulk electrolyte solution systems as used in the DFTMD simulations (for the deprotonation free energy calculation). The extended simple point-charge (SPC/E) water model 26 and Joung-Cheatham (JC) ion models were used in these additional classical MD simulations, which have been validated before in terms of both thermodynamic and transport properties at higher salt concentration. Subsequently, the Poisson potentials were calculated with both DFT and point-charge models, using a method summarized in Fig. The main results from DFTMD simulations of the deprotonation free energy and the Poisson potential as well as the proton work function and the water dissociation constant are listed in Table . Both the deprotonation free energy ∆ dp A H 3 O + and the Poisson po-tential ϕ increase with the salt concentration. However, the relative shift is larger in the case of the Poisson potential than the corresponding deprotonation free energy. As a results, the proton work function W H + goes down rather than going up in concentrated electrolyte solutions. Therefore, the equilibrium potential of HER under the acidic conditions becomes more positive. This confirm previous experimental observations of an increased proton concentration in concentrated electrolyte solutions. However, the situation is different for the HER activity under the alkaline condition. The water dissociation constant pK w becomes larger with the salt concentration. Following Eq. 5, this positive shift in the water dissociation constant can counterbalance the negative shift in the proton work function. Similar ion enhancement effects the pK a have been also observed for silanol group. Interestingly, this thermodynamic balance appeared to be tipped to the side of a reduced HER activity, as recently reported with a similar computational methodology but for highly concentrated LiTFSI aqueous solutions. Therefore, our result suggest that one may tune the pK a of buffer molecules to regulate the HER activity in concentrated electrolyte solutions. Under either acidic or alkaline conditions, the Poisson potential contributes a major part to the shift in the HER potential in concentrated electrolyte solutions. Therefore, it is worth a further discussion about its physical significance. Many computational studies have been performed to determine single-ion solvation free energies and the closely related Poisson potential of the water-air interface. Water-air interfacial potential calculated with DFT takes on values of 3-4 V. In contrast, the water-air interfacial potential of aqueous solutions calculated with classical point-charge models produce potentials one order of magnitude smaller and with a negative sign. Furthermore, it has been established that the water-air interfacial potential cannot be constructed from molecular dipole moments alone but require a significant contribution from the quadrupolar terms. Such a partitioning of the water-air interface potential was performed for DFT models in ref. by Leung. Using maximally localized Wannier functions, a partition into multipole moments was performed,