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Ice-water, water-vapor interfaces and ice surface are studied by moleculardynamics simulations with the SPC/E model of water molecules having the purposeto estimate the profiles of electrostatic potential across the interfaces. Wehave proposed a methodology for calculating the profiles of electrostaticpotential based on a trial particle, which showed good agreement for the caseof electrostatic potential profile of the water-vapor interface of TIP4P modelcalculated in another way. The measured profile of electrostatic potential forthe pure ice-water interface decreases towards the liquid bulk region, which isin agreement with simulations of preferential direction of motion of Li$^{+}$and F$^{-}$ solute ions at the liquid side of the ice-water interface. Theseresults are discussed in connection with the Workman-Reynolds effect.

Non-monotonous changes in velocity autocorrelations across the transformation from molecular to atomic fluid in hydrogen under pressure are studied by ab initio molecular dynamics simulations at the temperature 2500 K. We report diffusion coefficients in a wide range of densities from purely molecular fluid up to metallic atomic fluid phase. An analysis of contributions to the velocity autocorrelation functions from the motion of molecular centers-of-mass, rotational and intramolecular vibrational modes is performed, and a crossover in the vibrational density of intramolecular modes across the transition is discussed. [ABSTRACT FROM AUTHOR]

The collective dynamics in liquid water is an active research topic experimentally, theoretically and via simulations. Here, ab initiomolecular dynamics simulations are reported in heavy and ordinary water at temperature 323.15 K, or 50°C. The simulations in heavy water were performed both with and without dispersion corrections. We found that the dispersion correction (DFT-D3) changes the relaxation of density-density time correlation functions from a slow, typical of a supercooled state, to exponential decay behaviour of regular liquids. This implies an essential reduction of the melting point of ice in simulations with DFT-D3. Analysis of longitudinal (L) and transverse (T) current spectral functions allowed us to estimate the dispersions of acoustic and optic collective excitations and to observe the L-T mixing effect. The dispersion correction shifts the L and T optic (O) modes to lower frequencies and provides by almost thirty per cent smaller gap between the longest-wavelength LO and TO excitations, which can be a consequence of a larger effective high-frequency dielectric permittivity in simulations with dispersion corrections. Simulation in ordinary water with the dispersion correction results in frequencies of optic excitations higher than in D2O, and in a long-wavelength LO-TO gap of 24 ps-1(127 cm-1). [ABSTRACT FROM AUTHOR]

High-resolution inelastic x-ray scattering measurements were carried outon molten NaInearthe melting point at 680c'C at SPring-8. Small and damped indications of longitudinal optic excitation modes were observed on the tails of the longitudinal acoustic modes at small momentum transfers, Q ~ 5 nm-1. The measured spectra are in good agreement, in both frequencyand linewidth, with ab initio molecular dynamics (MD) simulations but not classical MD simulations. The observation of these modes at small Q and a good agreement with the simulation permits clear identification of these as collective optic modes with well defined phasing between different ionic motions. [ABSTRACT FROM AUTHOR]

A simple ansatz for the study of velocity autocorrelation functions in fluids at different timescales is proposed. The ansatz is based on an effective summation of the infinite continued fraction at a reasonable assumption about convergence of relaxation times of the higher order memory functions, which have a purely kinetic origin. The VAFs obtained within our approach are compared with the results of the Markovian approximation for memory kernels. It is shown that although in the "overdamped" regime both approaches agree to a large extent at the initial and intermediate times of the system evolution, our formalism yields power law relaxation of the VAFs which is not observed at the description with a finite number of the collective modes. Explicit expressions for the transition times from kinetic to hydrodynamic regimes are obtained from the analysis of the singularities of spectral functions in the complex frequency plane. [ABSTRACT FROM AUTHOR]