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 5Å 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.

We develop an efficient Ewald method of molecular dynamics simulation for calculating the electrostatic interactions among charged and polar particles between parallel metallic plates, where we may apply an electric field with an arbitrary size. We use the fact that the potential from the surface charges is equivalent to the sum of those from image charges and dipoles located outside the cell. We present simulation results on boundary effects of charged and polar fluids, formation of ionic crystals, and formation of dipole chains, where the applied field and the image interaction are crucial. For polar fluids, we find a large deviation of the classical Lorentz-field relation between the local field and the applied field due to pair correlations along the applied field. As general aspects, we clarify the difference between the potential-fixed and the charge-fixed boundary conditions and examine the relationship between the discrete particle description and the continuum electrostatics.

We propose a simple microscopic model of molecular dynamics simulation to study orientational glass in three dimensions. We present simulation results for mixtures of mildly anisotropic particles and spherical impurities. We realize fcc solids without orientational order in a rotator phase. As the temperature T is lowered, the disordered matrix is gradually replaced by four kinds of orientationally ordered, rhombohedral domains. Two-phase coexistence is realized in a temperature window. The impurities serve to anchor the orientations of the surrounding anisotropic particles, resulting in finely divided domains or medium long-range orientational order. We examine the rotational dynamics of the molecular orientations which is slowed down at low T . We predict the shape memory effect under a stretching cycle due to inter-variant transformation.

We examine the thermal fluctuations of the local electric field E loc k and the dipole moment µ k in liquid water at T = 298 K between metal walls in electric field applied in the perpendicular direction. We use analytic theory and molecular dynamics simulation. In this situation, there is a global electrostatic coupling between the surface charges on the walls and the polarization in the bulk. Then, the correlation function of the polarization density pz(r) along the applied field contains a homogeneous part inversely proportional to the cell volume V . Accounting for the longrange dipolar interaction, we derive the Kirkwood-Fröhlich formula for the polarization fluctuations when the specimen volume v is much smaller than V . However, for not small v/V , the homogeneous part comes into play in dielectric relations. We also calculate the distribution of E loc k in applied field. As a unique feature of water, its magnitude |E loc k | obeys a Gaussian distribution with a large mean value E0 ∼ = 17 V/nm, which arises mainly from the surrounding hydrogen-bonded molecules. Since |µ k |E0 ∼ 30kBT , µ k becomes mostly parallel to E loc k . As a result, the orientation distributions of these two vectors nearly coincide, assuming the classical exponential form. In dynamics, the component of µ k (t) parallel to E loc k (t) changes on the timescale of the hydrogen bonds ∼ 5 ps, while its smaller perpendicular component undergoes librational motions on timescales of 0.01 ps.