1,721,037 research outputs found
LIBRATIONAL DYNAMICS OF WATER IN TERMS OF ANGULAR VELOCITY CORRELATION-FUNCTIONS AND ORIENTATIONAL STRUCTURE
No full textThe high frequency (librational) dynamics of water is studied by molecular dynamics simulation on the TIP4P model at 245 K. The single-molecule part of the hydrogen current is described by a combination of autocorrelation functions of components of the angular velocity along the principal axes of inertia of the molecules. The distinct part of the hydrogen current is represented by a sum of cross correlation functions of the same components. The spectrum of the self current is almost quantitatively reproduced by the sum of the spectra of two of these contributions. The first is that relative to the component along the axis normal to the molecular plane and the second along the axis normal to the dipole vector, in the molecular plane, both multiplied by a geometrical parameter of the molecule. In the case of the distinct part, the interparticle correlation function of the projection of the angular velocity over the same two axes is weighted by the dot product of the dipole moments of each pair of molecules. The agreement between the results obtained this way and according to the usual definition is qualitatively satisfactory. The axis normal to the dipole in the plane of the molecule turns out to be a favourite channel for the propagation of rotational correlations in the liquid. The analysis of hydrogen current in terms of rotational correlations allows to show that the transition from single-molecule to collective dynamics is essentially complete when clusters of two or three shells of neighbors are considered. A simple model of the distinct hydrogen current as a time-propagated self function is able to qualitatively account for all features of the spectra of both longitudinal and transverse current. The ratio of the transverse to longitudinal delay time is shown to be equal to the square root of the amplitude of the librational band of the dielectric spectrum
THE FREQUENCY AND WAVELENGTH DEPENDENT DIELECTRIC PERMITTIVITY OF WATER
No full textThe static and dynamic dielectric behaviour of a model of liquid water is studied in the supercooled region, as a function of wavelength. The data used for this analysis were obtained in a previous molecular dynamics simulation of a sample of 343 water molecules, modelled by the TIP4P potential, at 245 K. A substantial wavelength dependence of the static dielectric permittivity is observed, while the value found for epsilon(0) seems to indicate that the TIP4P model is able to account qualitatively for the increase of dielectric constant upon temperature decrease, as found in real water. The time correlation function of the longitudinal and transverse components of the dipole density as well as that of the individual and total dipole moment is also calculated, to relate collective dielectric properties to the single molecule relaxation. The collective and single molecule dielectric relaxation times are obtained and their ratio seems to be close to a static property, namely the short-range orientational correlation factor, g(s)(k). The frequency dependence of the dielectric constant is compared with previous simulation and experimental values. Furthermore, the longitudinal and transverse components of the hydrogen current are discussed. The molecular symmetry relates the latter properties to the corresponding component of the dipole density in the frequency range 80-200 THz, typical of the librational motions of water. From these results, a simple and unifying picture of the dynamics underlying the three dielectric bands of water in the frequency range 1-200 THz emerges
STRESS TENSOR AND VISCOSITY OF WATER - MOLECULAR-DYNAMICS AND GENERALIZED HYDRODYNAMICS RESULTS
No full textThe time correlation functions (CF's) of diagonal and off-diagonal components of the stress tenser of water have been calculated at 245 and 298 K in a molecular dynamics (MD) study on 343 molecules in the microcanonical ensemble. We present results obtained at wave number k = 0 and at a few finite. values of k, in the atomic and molecular formalism. In all cases; more than 98% of these functions are due to the potential term of the stress tenser. At k = 0, their main features are a fast oscillatory initial decay, followed by a long-time tail more apparent in the supercooled region. Bulk and shear viscosities, calculated via Green-Kubo integration of the relevant CF at k = 0, are underestimated with respect to experimental data, mainly at low temperature, but their ratio (approximate to 2)is correctly reproduced. Both shear and bulk viscosity decrease as a function of k, the latter more rapidly, so that they become almost equal at approximate to 1 Angstrom(-1) Also, both viscosities drop rapidly from their maximum at omega = 0. This behavior has been related to the large narrowing observed in the acoustic band, mainly in the supercooled region. The infinite frequency bulk and shear rigidity moduli have been shown to be in fair agreement with the experimental data, provided the MD value used for comparison is that corresponding to the frequency range relevant to ultrasonic measurements. The MD results of stress-stress CF's compare well with those predicted by Bertolini and Tani [Phys. Rev. E 51, 1091 (1995)] at k = 0, by an application of generalized hydrodynamics [de Schepper et al., Phys. Rev. A 38, 271 1988)] in the molecular formalism, to the same model of water (TIP4P) [Jorgensen et al., J. Chem. Phys. 79, 926 (1983)]. These CF's are essentially equal in the: atomic and molecular formalism, the only minor difference being restricted to the high frequency librational region of the shear function. By a comparison of atomic and molecular results, we show here that neglecting libration has no effect on the density-density and longitudinal current CF's and very little effect on transverse properties. On the other hand, this study points out the importance of including the oscillation in the nearest-neighbor cage in the memory function of the longitudinal and transverse current CF. The oscillatory local motion turns out to play an important role in all CF's and hence contributes significantly to the value of viscosity and of rigidity moduli
Thermal conductivity of water: Molecular dynamics and generalized hydrodynamics results
No full textEquilibrium molecular dynamics simulations have been carried out in the microcanonical ensemble at 300 and 255 K on the extended simple point charge (SPC/E) model of water [Berendsen et al., J. Phys. Chem. 91, 6269 (1987)]. In addition to a number of static and dynamic properties, thermal conductivity lambda has been calculated via Green-Kubo integration of the heat current time correlation functions (CF's) in the atomic and molecular formalism, at wave number k=0. The calculated values (0.67 +/- 0.04 W/mK at 300 K and 0.52 +/- 0.03 W/mK at 255 K) are in good agreement with the experimental data (0.61 W/mK at 300 K and 0.49 W/mK at 255 K). A negative long-time tail of the heat current CF, more apparent at 255 K, is responsible for the anomalous decrease of lambda with temperature. An analysis of the dynamical modes contributing to lambda has shown that its value is due to two low-frequency exponential-like modes, a faster collisional mode, with positive contribution, and a slower one, which determines the negative long-time tail. A comparison of the molecular and atomic spectra of the heat current CF has suggested that higher-frequency modes should not contribute to lambda in this temperature range. Generalized thermal diffusivity D-T(k) decreases as a function of k, after an initial minor increase at k = k(min). The k dependence of the generalized thermodynamic properties has been calculated in the atomic and molecular formalisms. The observed differences have been traced back to intramolecular or intermolecular rotational effects and related to the partial structure functions. Finally, from the results we calculated it appears that the SPC/E model gives results in better agreement with experimental data than the transferable intermolecular potential with four points TIP4P water model [Jorgensen et al., J. Chem. Phys. 79, 926 (1983)], with a larger improvement for, e.g., diffusion, viscosities, and dielectric properties and a smaller one for thermal conductivity. The SPC/E model shares, to a smaller extent, the insufficient slowing down of dynamics at low temperature already found for the TIP4P water model
GENERALIZED HYDRODYNAMICS AND THE ACOUSTIC MODES OF WATER - THEORY AND SIMULATION RESULTS
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COLLECTIVE MOTION AND INTERPARTICLE CORRELATIONS IN LIQUID WATER - A MOLECULAR-DYNAMICS SIMULATION
No full textThe intramolecular and intermolecular contributions to the longitudinal current fluctuations of the hydrogens of liquid water have been computed by molecular dynamics simulation on the TIP4P model at 245 K. This indicates that the recently discovered high frequency (approximately 165 THz) 'optical-like' collective mode of the hydrogens is due to the intermolecular contributions and that these and the intramolecular part cancel each other in the frequency range 20-120 THz. Conversely, they mutually reinforce outside this region to produce the acoustical (approximately 8 THz) and 'optical like' collective bands, in the low-k region. The high frequency mode is rooted in the librational motion of the molecules. This is shown by a comparison of the intermolecular part spectrum with that of the interparticle correlation function of the projection of the angular velocity over the principal axes of inertia of the molecule. In particular, we are able to show that the high frequency band is almost entirely due to correlations of the rotation of the tagged molecule and that of the cluster of neighbours around the axis normal to the dipole, in the molecular plane. Large cage effects are also observed in the translational dynamics, and the interparticle centre of mass velocity correlation function is found to be almost equal to the time propagated auto-correlation function
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
On the generalized bulk viscosity behavior
No full textWe have calculated generalized longitudinal, eta (L)(k), shear, eta (S)(k), and bulk, eta (B)(k) viscosity of argon at two thermodynamic states by molecular-dynamics simulation. Bulk viscosity has been evaluated with two independent methods that give results in good agreement. We find that bulk viscosity stays positive at all wave numbers k, and show that negative values of eta (B)(k) are due to a straightforward (and incorrect) extension to finite k's of the relation eta (B)(k) = eta (L)(k)-4 eta (S)(k)/3, only valid at vanishing k's
Il piacere della chiarezza e della brevità negli scritti di Giuseppe Pontiggia
No full textGiuseppe Pontiggia è tra i maggiori scrittori del Novecento. Egli è un maestro di chiarezza e di concisione. Nella sua scrittura egli coniuga limpidezza e complessità. Nello spazio limitato dell'aforisma e in quello più dilatato ma sempre contenuto del saggio Pontiggia misura le parole in estensione perché siano più efficaci in profondità. L'obiettivo è il massimo contenuto nel minimo volume
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