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Molecular Dynamics Simulation of Water between Metal Walls under Electric Field: Dielectric Response and Dynamics after Field Reversal

We study water between parallel metal walls under applied electric field accounting for the image effect at $T=298$ K. The electric field due to the surface charges serves to attract and orient nearby water molecules, while it tends to a constant determined by the mean surface charge density away from the walls. We find Stern boundary layers with thickness about $5$ $\rm Å$ and a homogeneously polarized bulk region. The molecules in the layers more sensitively respond to the applied field than in the bulk. As a result, the potential drop in the layers is larger than that in the bulk unless the cell length exceeds 10 nm. We also examine the hydrogen bonds, which tend to make small angles with respect to the walls in the layers even without applied field. The average local field considerably deviates from the classical Lorentz field and the local field fluctuations are very large in the bulk. If we suppose a nanometer-size sphere around each molecule, the local field contribution from its exterior is nearly equal to that from the continuum electrostatics and that from its interior yields the deviation from the classical Lorentz field. As a nonequilibrium problem, we investigate the dynamics after a reversal of applied field, where the relaxation is mostly caused by large-angle rotational jumps after 1 ps due to the presence of the hydrogen bond network. The molecules undergoing these jumps themselves form hydrogen-bonded clusters heterogeneously distributed in space.