Effect of the Berendsen thermostat on dynamical properties of water

The effect of the Berendsen thermostat on the dynamical properties of bulk SPC/E water is tested by generating power spectra associated with fluctuations in various observables. The Berendsen thermostat is found to be very effective in preserving temporal correlations in fluctuations of tagged particle quantities over a very wide range of frequencies. Even correlations in fluctuations of global properties, such as the total potential energy, are well-preserved for time periods shorter than the thermostat time constant. Deviations in dynamical behaviour from the microcanonical limit do not, however, always decrease smoothly with increasing values of the thermostat time constant but may be somewhat larger for some intermediate values of $\tau_B$, specially in the supercooled regime, which are similar to time scales for slow relaxation processes in bulk water.Comment: 21 pages, 5 figures, To be published in Mol. Phy

Signatures of multiple time-scale behaviour in the power spectra of water

Power spectra associated with fluctuations in the tagged particle potential and kinetic energies are analysed for bulk SPC/E water for a range of temperatures along the 1.0 g/cm3 isochore. Fluctuations in the tagged particle potential energies give rise to 1/fα noise, indicative of multiple time-scale behaviour, over a temperature-dependent frequency regime. In contrast, the tagged particle centre-of-mass and rotational kinetic energies, which are indicative of the magnitude of local thermal fluctuations, do not show any evidence of 1/fα behaviour

Spectral Signatures of the Diffusional Anomaly in Water

Analysis of power spectrum profiles for various tagged particle quantities in bulk SPC/E water is used to demonstrate that variations in mobility associated with the diffusional anomaly are mirrored in the exponent of the \onebyf\ region. Monitoring of \onebyf behaviour is shown to be a simple and direct method for linking phenomena on three distinctive length and time scales: the local molecular environment, hydrogen bond network reorganisations and the diffusivity. The results indicate that experimental studies of supercooled water to probe the density dependence of $1/f^\alpha$ spectral features, or equivalent stretched exponential behaviour in time-correlation functions, will be of interest.Comment: 5 Pages, 4 Figure

Effect of ionic solutes on the hydrogen bond network dynamics of water: power spectral analysis of aqueous NaCl solutions

To understand the modifications of the hydrogen bond network of water by ionic solutes, power spectra as well as static distributions of the potential energies of tagged solvent molecules and solute ions have been computed from molecular dynamics simulations of aqueous NaCl solutions. The key power spectral features of interest are the presence of high-frequency peaks due to localized vibrational modes, the existence of a multiple time scale or 1/ƒ α frequency regime characteristic of networked liquids, and the frequency of crossover from 1/ƒ α type behavior to white noise. Hydrophilic solutes, such as the sodium cation and the chloride anion, are shown to mirror the multiple time scale behavior of the hydrogen bond network fluctuations, unlike hydrophobic solutes which display essentially white noise spectra. While the power spectra associated with tagged H<SUB>2</SUB>O molecules are not very sensitive to concentration in the intermediate frequency 1/ƒ α a regime, the crossover to white noise is shifted to lower frequencies on going from pure solvent to aqueous alkali halide solutions. This suggests that new and relatively slow time scales enter the picture, possibly associated with processes such as migration of water molecules from the hydration shell to the bulk or conversion of contact ion pairs into solvent-separated ion pairs which translate into variations in equilibrium transport properties of salt solutions with concentration. For anions, cations, and solvent molecules, the trends in the a exponents of the multiple time scale region and the self-diffusivities are found to be strongly correlated

Multiple time-scale behavior of the hydrogen-bond network in water

The temperature-dependent changes in the hydrogen-bond network of SPC/E water have been examined using power spectral analysis of fluctuations in tagged-molecule potential energies and local tetrahedral order parameters. The clear signatures of multiple time-scale or 1/ƒ α behavior in the power spectra are shown to depend sensitively on the strength of hydrogen bonding. The analysis focuses on three specific power spectral features:the frequency of crossover to white noise behavior, the exponent in the 1/ƒ α regime, and the librational peak. The exponent of the tagged-particle potential-energy fluctuations is shown to be strongly correlated with the diffusivity in the temperature range of 230 to 300 K. This correlation is strongest in the temperature-density regimes where the mechanism for diffusion is likely to be dominated by translational-rotational coupling, suggesting that the value of the exponent is a measure of the efficiency of the coupling of librational modes with network vibrations. The temperature dependence of all power spectral features was found to be strongest along the 0.9-g cm-3 isochore, which corresponds closely to the density of minimum diffusivity for the temperature range studied here. The static distributions of the tagged-particle quantities were examined to determine the degree of heterogeneity of the local molecular environment and its relationship with power spectral features

Diffusional anomaly and network dynamics in liquid silica

The present study applies the power spectral analysis technique to understand the diffusional anomaly in liquid silica, modeled using the Beest-Kramer-van Santen (BKS) potential. Molecular-dynamics simulations have been carried out to show that power spectrum of tagged particle potential energy of silica shows a regime with 1/f&#945; dependence on frequency f which is the characteristic signature of multiple time scale behaviour in networks. As demonstrated earlier in the case of water [ J. Chem. Phys. 122, 104507 (2005) ], the variations in the mobility associated with the diffusional anomaly are mirrored in the scaling exponent &#945; associated with this multiple time scale behavior. Our results indicate that in the anomalous regime, as the local tetrahedral order decreases with temperature or pressure, the coupling of local modes to network reorganizations increases and so does the diffusivity. This symmetry-dependence of the vibrational couplings is responsible for the connection between the structural and diffusional anomalies

Spectral characterization of hydrogen bond network dynamics in water

3nonemixedA. MUDI; C. CHAKRAVARTI; MILOTTI E.A., Mudi; C., Chakravarti; Milotti, Edoard