Umberto Balucani

Analysis of the velocity autocorrelation function of water

The centre-of-mass velocity autocorrelation function of liquid water under room conditions is examined through a mode-coupling approach developed to study monatomic liquids. The wavevector-dependent current correlations relevant to the theory are evaluated by computer simulations using a recently developed interaction potential, which accounts for the polarizability of the molecule. The theory is found to reproduce all the relevant dynamical features of water. Moreover, the analysis in terms of longitudinal and transverse contributions clarifies the origin of some peculiarities in the dynamics of this single-molecule correlation. A discussion of the corresponding spectra is also given.

Interparticle velocity correlations in simple liquids

The momentum transfer of a test particle to a cluster of atoms initially in the first shell of neighbors is investigated at liquid densities by molecular dynamics methods. The comparison of the results obtained for Lennard‐Jones and soft spheres models is discussed. In the short time regime the interpretation of the data is shown to parallel that of the single particle velocity autocorrelation function. At longer times, a simple physical model can account for the decay of the cross correlation in terms of the increasing number of atoms involved in the process.

Velocity correlations, cooperative effects and relative diffusion in simple liquids

A study of momentum transfer in liquids has been carried out by computer simulation of the process in a Lennard-Jones fluid. The data have also been compared with theoretical predictions. The conclusions emphasise the importance of understanding cooperative dynamical effects amongst nearest and next-nearest neighbours when discussing single-particle motion. The authors also investigate the relative diffusion of a pair of atoms that are initially nearest neighbours, and show how cooperative effects slow the relative motion.

VELOCITY CORRELATIONS IN LIQUID HYDROGEN FLUORIDE

The center-of-mass velocity autocorrelation function is analyzed by computer simulation in a model of liquid hydrogen fluoride at two state points. In comparison with water (another hydrogen-bonded liquid) new features arise. To understand the peculiarities of HF, we have investigated atomic velocity correlations in both the laboratory and a molecular frame. The comparison of the frequency spectra permits to ascertain the role of fluorine–hydrogen correlations (or of rototranslational couplings) in the center-of-mass velocity autocorrelation function. At low temperature, the appearance of a long time tail is discussed in terms of projections in the two references frames, and found to be mostly associated with orientational correlations. A discussion in terms of velocity transfer between nearest-neighbor molecules is also given.

Evolution from ordinary to fast sound in water at room temperature

Extensive computer simulations are performed at room temperature in liquid D2O modelled in terms of a realistic intermolecular potential, with the purpose of providing a detailed account of the dispersion of longitudinal current correlations in this system. The data support the results of a recent theoretical study, where a preliminary microscopic interpretation of the anomalous dispersion was proposed. A deeper memory-function analysis shows that a combined kinetic and mode-coupling framework can qualitatively account for the basic dynamical features of the phenomenon.

A self-consistent theory of single-particle motion in ordinary and supercooled liquids

The mean square displacement of a tagged particle in a liquid is known to exhibit a diffusive linear time dependence beyond a microscopic timescale. By making use of simple mode-coupling concepts the authors derive a set of analytic self-consistent equations for the relevant dynamical quantities in this regime, namely the diffusion coefficient and the intercept. The results of the theory are successfully compared with the data obtained by simulation experiments in different systems.

Self-diffusion and relative diffusion processes in liquids: microdynamic and hydrodynamic points of view

A microdynamic theory for the self-diffusion coefficient, which involves a wave-number-dependent viscosity, eta (q), is successfully tested on a hard-sphere system. The models the authors introduce for eta (q) are adapted to the Lennard-Jones fluid. New molecular dynamics data for the relative diffusion coefficient, in which the initial separation of two particles appears as an additional degree of freedom, are reported for this fluid. A systematic investigation of near-neighbour pairs shows (i) clear evidence of effects that slow their relative motion and (ii) that the relative diffusion coefficient varies remarkably smoothly with separation. The theory is extended to interpret the results and it is shown that the expressions for both types of diffusion coefficient take a form that is strongly reminiscent of a hydrodynamic treatment. This, the authors suggest, is why hydrodynamic theories (e.g. the Stokes-Einstein equation for the self-diffusion coefficient, and the Oseen approach to relative diffusion) appear to be applicable even at an atomic level. They emphasise, instead, the essentially microscopic nature of the diffusion process in monatomic liquids.

Wavevector-Dependent Shear Viscosity in Lennard-Jones Liquids

Computer simulation data for the generalized wavevector-dependent shear viscosity η(q) are reported for the first time in a Lennard-Jones fluid. The transverse current autocorrelation functions are also presented. The results are carefully compared with simple viscoelectric theory. Although the latter is found to give a respectable description of the data, discrepancies are pointed out.

Collective dynamics of liquid HCl: The density–density and longitudinal current correlations

In this work the dynamics of density fluctuations in liquid HCl is investigated by computer simulation experiments, with the main goal of ascertaining the influence of hydrogen bonding in the features of the collective excitations of this molecular fluid. The data analysis shows that in HCl the hydrogen bonding has quite a small relevance on the dynamics, in strong contrast with the findings reported for both HF and water. Within the framework of generalized hydrodynamics we have been able to derive values for otherwise unknown quantities like the ratio of specific heats and the adiabatic sound velocity. An evaluation of the average effective interaction potential between the molecular centers of mass, clarifies the interpretation of the collective dynamical behavior explored in the present investigation.

Transport properties of liquid hydrogen fluoride

The dynamical properties of liquid hydrogen fluoride are investigated by a molecular dynamics study of the correlation functions relevant for a generalized hydrodynamics description of transport coefficients. The results are compared with the corresponding ones in liquid water in order to understand the role of hydrogen bonding in the two systems. The different behavior can ultimately be attributed to the arrangement of the molecules, which form irregular chains in HF and a tetrahedral network in water. For the two systems, the differences between experimentally measurable quantities are also pointed out and discussed.