The microscopic origin of the surface potential has been examined by Harder and Roux 20 by using atomistic simulations. They have shown that the interface potential arises not due to orientational ordering of molecules on the surface, but due to the intramolecular asymmetry of the charge distribution. By using molecular dynamics (MD) simulations, they estimated the interface potential of water to be equal to -510 meV. The negative sign means that the solvation energy of anions becomes less negative and the solvation energy of cations becomes more negative due to the interface (as per Eq. ( )), and that the apparent re with a more realistic simulation of a interface, the IP model whose center coincides etric center of the TIP3P force field ͑i.e., IPsen. This choice is based on the simple ratioules at the interface prefer to be solvated by olecules and will therefore tend to minimize surface area to vacuum. The realistic vacuumsystem is generated from a MD simulation of ter molecules forming a liquid slab surrounded e molecule and charge density from the simu-IP3b model are presented in the top panels of 3 shows the corresponding potential profile. drop for IP-TIP3Pb is 400 mV, in reasonable h the MD simulation value of 520 mV. from Fig. that varying the center of the IP n the geometric center and the oxygen atom ad to precise agreement with the interface poted from the MD simulation. However, this that the orientational structure of water molcuum-water interface system is purely isotroumed by construction in the IP model. Experihown that water molecules adopt a subtle rder in the interface region between the liquid This orientational structure does not go away riate choice of molecular center but the cumution to the IP does go to zero. om these atomistic force fields of water and omic charge densities, Z i . This core atom elec-nearly an order of magnitude larger in magnitude and of opposite sign to the potentials calculated from the IP-TIP3P model or the associated TIP3P force field. Such a large posi- The negative sign of χ is consistent with the energy-level diagram in Fig. .