Generalised hydrodynamic description of the time correlation functions of liquid metals:<i>ab initio</i>molecular dynamics study

The Journal of Chemical Physics

Ruocco

Scopigno

et al. 2015

Self Cite

Unlike phonons in crystals, the collective excitations in liquids cannot be treated as propagation of harmonic displacements of atoms around stable local energy minima. The viscoelasticity of liquids, reflected in transition from the adiabatic to elastic high-frequency speed of sound and in absence of the long-wavelength transverse excitations, results in dispersions of longitudinal (L) and transverse (T) collective excitations essentially different from the typical phonon ones. Practically, nothing is known about the effect of high pressure on the dispersion of collective excitations in liquids, which causes strong changes in liquid structure. Here dispersions of L and T collective excitations in liquid Li in the range of pressures up to 186 GPa were studied by ab initio simulations. Two methodologies for dispersion calculations were used: direct estimation from the peak positions of the L/T current spectral functions and simulation-based calculations of wavenumber-dependent collective eigenmodes. It is found that at ambient pressure, the longitudinal and transverse dynamics are well separated, while at high pressures, the transverse current spectral functions, density of vibrational states, and dispersions of collective excitations yield evidence of two types of propagating modes that contribute strongly to transverse dynamics. Emergence of the unusually high-frequency transverse modes gives evidence of the breakdown of a regular viscoelastic theory of transverse dynamics, which is based on coupling of a single transverse propagating mode with shear relaxation. The explanation of the observed high-frequency shift above the viscoelastic value is given by the presence of another branch of collective excitations. With the pressure increasing, coupling between the two types of collective excitations is rationalized within a proposed extended viscoelastic model of transverse dynamics. Unlike phonons in crystals, the collective excitations in liquids cannot be treated as propagation of harmonic displacements of atoms around stable local energy minima. The viscoelasticity of liquids, reflected in transition from the adiabatic to elastic high-frequency speed of sound and in absence of the long-wavelength transverse excitations, results in dispersions of longitudinal (L) and transverse (T) collective excitations essentially different from the typical phonon ones. Practically, nothing is known about the effect of high pressure on the dispersion of collective excitations in liquids, which causes strong changes in liquid structure. Here dispersions of L and T collective excitations in liquid Li in the range of pressures up to 186 GPa were studied by ab initio simulations. Two methodologies for dispersion calculations were used: direct estimation from the peak positions of the L/T current spectral functions and simulation-based calculations of wavenumber-dependent collective eigenmodes. It is found that at ambient pressure, the longitudinal and transverse dynamics are well separated, while at high pressures, the...

Section: Theoretical Analysis Of Collective Dynamicsmentioning

confidence: 99%

Pressure-induced emergence of unusually high-frequency transverse excitations in a liquid alkali metal: Evidence of two types of collective excitations contributing to the transverse dynamics at high pressures

The Journal of Chemical Physics

Ruocco

Scopigno

et al. 2015

Self Cite

“…One can see perfect recovering by the thermo-viscoelastic theory the damping oscillations in the time dependence of current correlations. As it was stated in [23] the nice quality of the fit is provided by the exact sum rules for short-time behavior and time moments of the corresponding AIMD-derived function. The proposed fitting scheme in that sense has advantages over the fitting of spectral functions with spectral shapes of a single or two damped harmonic oscillators (DHO) [26].…”

Section: Resultsmentioning

confidence: 99%

“…In particular, in [23] a thermo-viscoelastic analysis based on generalized collective modes (GCM) approach in connection with the ab initio simulations was suggested. In the thermo-viscoelastic approach a five-variable dynamic model is considered:…”

Section: Jj (K T)mentioning

confidence: 99%

“…An important issue is the number of exact sum rules fulfilled for theoretical time correlation functions (short-time behavior), and the thermo-viscoelastic dynamic model provides the same first time derivatives at the origin up to the fourth order for density-density time correlations functions from theory and simulations, fulfilling in general six sum rules within the GCM scheme. For the longitudinal current-current time correlation functions this GCM model provides six sum rules too [23].…”

Section: Jj (K T)mentioning

confidence: 99%

“…The set of dynamic variables A (5) (k, t) is applied for solving the generalized Langevin equation [1] in matrix form in terms of dynamic eigenmodes for so-called generalized hydrodynamic matrix T(k) [24]. The corresponding eigenvectors allow to estimate contribution of particular dynamic mode to the time correlation function of interest, and the quality of the theory is usually verified by comparison of theoretical and simulation-derived time correlation functions [23,25]. An important issue is the number of exact sum rules fulfilled for theoretical time correlation functions (short-time behavior), and the thermo-viscoelastic dynamic model provides the same first time derivatives at the origin up to the fourth order for density-density time correlations functions from theory and simulations, fulfilling in general six sum rules within the GCM scheme.…”

Section: Jj (K T)mentioning

confidence: 99%

See 2 more Smart Citations

Ab initio molecular dynamics study of collective dynamics in liquid Tl: Thermo-viscoelastic analysis

Demchuk

2017

EPJ Web Conf.

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Abstract.We studied collective dynamics of pure liquid metal Tl using a combination of ab initio molecular dynamics (AIMD) simulations and a thermoviscoelastic model applied to calculations of dynamic eigenmodes and dispersion of collective excitations in particular. We found that for liquid Tl at ambient pressure the transverse current spectral functions obtained directly in ab initio simulations for wave numbers larger than first pseudo-Brillouin-zone boundary contain two low-and high-frequency peaks that is an evidence of emergence of the unusually high-frequency transverse modes as it was observed before in liquid Li at very high pressures. The thermo-viscoelastic dynamic model shows perfect reproduction of the simulation-derived longitudinal current autocorrelation functions, and the acoustic eigenmodes are in nice agreement with the peaks of the longitudinal current spectral functions up to the first pseudoBrillouin-zone boundary. The deviation of the dynamic eigenmodes from peak positions at higher wave numbers gives evidence of L-T coupling effects.

Collective Excitations in Supercritical Fluids

Springer Proceedings in Physics

Gorelli

Ruocco

et al. 2015

Self Cite

Recent progress in theoretical and simulation studies of collective excitations in supercritical fluids is reviewed. We discuss a methodology of fit-free estimation of dispersion of longitudinal and transverse excitations in simple fluids. The issue of vanishing positive sound dispersion-a viscoelastic increase of the speed of sound from adiabatic one to its high-frequency (elastic) value-with reduction of density, as it was observed in inelastic X-ray scattering experiments on supercritical Ar, is discussed from the point of view of finding distinctions in collective dynamics of low-and high-density supercritical fluids. On the basis of several theoretical models within the extended hydrodynamic description of density-density correlations in liquids analytical expressions for positive sound dispersion are obtained and applied for analysis of time correlation functions obtained from molecular dynamics simulations. A location of a crossover from the liquid-like to gas-like types of collective dynamics is discussed based on general findings for spectra of collective excitations in supercritical Ar and soft sphere fluids.