The Experts below are selected from a list of 3768 Experts worldwide ranked by ideXlab platform
Stefano Bellissima - One of the best experts on this subject based on the ideXlab platform.
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density dependence of the dynamical processes governing the Velocity Autocorrelation Function of a quantum fluid
Physical Review E, 2019Co-Authors: E Guarini, Stefano Bellissima, Daniele Colognesi, M Neumann, U BafileAbstract:We present an exponential mode analysis of the dynamical processes determining the time behavior of the Kubo Velocity Autocorrelation Function (KVAF) of fluid para-${\mathrm{H}}_{2}$, as obtained by ring polymer molecular dynamics simulations at various fluid densities. The mechanisms contributing to the decay of the KVAF are thoroughly characterized at a slightly supercritical temperature, in a density interval ranging from the critical point to the fluid-solid transition. We show that the quantum nature of the system does not influence the specific phenomena and decay channels through which a loss in Velocity correlation takes place, since these are the same as found in classical fluids. Similarly, a dynamical crossover is observed with increasing density, signaling the onset of a transverse-like dynamics like in classical systems. We also investigate the effect of density on the processes contributing to the most relevant property of a quantum fluid, namely, the large values of the total and zero-point kinetic energy arising through the Heisenberg uncertainty principle.
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Density and time scaling effects on the Velocity Autocorrelation Function of quantum and classical dense fluid para-hydrogen.
The Journal of chemical physics, 2019Co-Authors: Stefano Bellissima, M Neumann, U Bafile, F Barocchi, Daniele Colognesi, E GuariniAbstract:We report the results of a ring polymer molecular dynamics study of the Kubo Velocity Autocorrelation Function of a quantum fluid as para-hydrogen aimed at the comparison with its classical counterpart. Quite different density conditions were considered for both the classical and quantum cases, in order to compare the two systems before and after the dynamical crossover typically undergone by the Velocity Autocorrelation Function (VAF) of fluids at densities around the triple point, where the shape of the Function changes from a monotonic to an oscillatory behavior with a negative minimum. A detailed study of the phase diagram of classical para-hydrogen was necessary for a reasonable choice of the classical states to be taken into consideration, in the spirit of the classical principle of corresponding states. The shape of the quantum and classical VAF was thoroughly analyzed, exhibiting at all studied densities clear differences that might be taken as evidence of quantum effects. We show that these differences are substantially reduced by applying a state-dependent time scaling with respect to a reference time identified with the inverse of the collision rate. An even better coincidence in shape is found by comparing the two systems at slightly non-corresponding reduced densities, suggesting that the quantum system behaves almost like the classical one, but at systematically less dense reduced states of the latter. We also find an unexpected and quite interesting density trend of the collision rate of both classical and quantum para-hydrogen, which accounts for the effectiveness of the scaling throughout the explored density range. The mean kinetic energy and the diffusion coefficients are also discussed in some detail.We report the results of a ring polymer molecular dynamics study of the Kubo Velocity Autocorrelation Function of a quantum fluid as para-hydrogen aimed at the comparison with its classical counterpart. Quite different density conditions were considered for both the classical and quantum cases, in order to compare the two systems before and after the dynamical crossover typically undergone by the Velocity Autocorrelation Function (VAF) of fluids at densities around the triple point, where the shape of the Function changes from a monotonic to an oscillatory behavior with a negative minimum. A detailed study of the phase diagram of classical para-hydrogen was necessary for a reasonable choice of the classical states to be taken into consideration, in the spirit of the classical principle of corresponding states. The shape of the quantum and classical VAF was thoroughly analyzed, exhibiting at all studied densities clear differences that might be taken as evidence of quantum effects. We show that these diffe...
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density and time scaling effects on the Velocity Autocorrelation Function of quantum and classical dense fluid para hydrogen
Journal of Chemical Physics, 2019Co-Authors: Stefano Bellissima, U Bafile, F Barocchi, Daniele Colognesi, M Neumann, E GuariniAbstract:We report the results of a ring polymer molecular dynamics study of the Kubo Velocity Autocorrelation Function of a quantum fluid as para-hydrogen aimed at the comparison with its classical counterpart. Quite different density conditions were considered for both the classical and quantum cases, in order to compare the two systems before and after the dynamical crossover typically undergone by the Velocity Autocorrelation Function (VAF) of fluids at densities around the triple point, where the shape of the Function changes from a monotonic to an oscillatory behavior with a negative minimum. A detailed study of the phase diagram of classical para-hydrogen was necessary for a reasonable choice of the classical states to be taken into consideration, in the spirit of the classical principle of corresponding states. The shape of the quantum and classical VAF was thoroughly analyzed, exhibiting at all studied densities clear differences that might be taken as evidence of quantum effects. We show that these differences are substantially reduced by applying a state-dependent time scaling with respect to a reference time identified with the inverse of the collision rate. An even better coincidence in shape is found by comparing the two systems at slightly non-corresponding reduced densities, suggesting that the quantum system behaves almost like the classical one, but at systematically less dense reduced states of the latter. We also find an unexpected and quite interesting density trend of the collision rate of both classical and quantum para-hydrogen, which accounts for the effectiveness of the scaling throughout the explored density range. The mean kinetic energy and the diffusion coefficients are also discussed in some detail.
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density of states and dynamical crossover in a dense fluid revealed by exponential mode analysis of the Velocity Autocorrelation Function
Physical Review E, 2017Co-Authors: Stefano Bellissima, M Neumann, E Guarini, U Bafile, F BarocchiAbstract:Extending a preceding study of the Velocity Autocorrelation Function (VAF) in a simulated Lennard-Jones fluid [Phys. Rev. E 92, 042166 (2015)PLEEE81539-375510.1103/PhysRevE.92.042166] to cover higher-density and lower-temperature states, we show that the recently demonstrated multiexponential expansion method allows for a full account and understanding of the basic dynamical processes encompassed by a fundamental quantity as the VAF. In particular, besides obtaining evidence of a persisting long-time tail, we assign specific and unambiguous physical meanings to groups of exponential modes related to the longitudinal and transverse collective dynamics, respectively. We have made this possible by consistently introducing the interpretation of the VAF frequency spectrum as a global density of states in fluids, generalizing a solid-state concept, and by giving to specific spectral components, obtained through the VAF exponential expansion, the corresponding meaning of partial densities of states relative to specific dynamical processes. The clear identification of a high-frequency oscillation of the VAF with the near-top excitation frequency in the dispersion curve of acoustic waves is a neat example of the power of the method. As for the transverse mode contribution, its analysis turns out to be particularly important, because the multiexponential expansion reveals a transition marking the onset of propagating excitations when the density is increased beyond a threshold value. While this finding agrees with the recent literature debating the issue of dynamical crossover boundaries, such as the one identified with the Frenkel line, we can add detailed information on the modes involved in this specific process in the domains of both time and frequency. This will help obtain a still missing full account of transverse dynamics, in both its nonpropagating and propagating aspects which are linked through dynamical transitions depending on both the thermodynamic states and the excitation wave vectors.
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density of states and dynamical crossover in a dense fluid revealed by exponential mode analysis of the Velocity Autocorrelation Function
arXiv: Statistical Mechanics, 2016Co-Authors: Stefano Bellissima, M Neumann, U Bafile, E Guarini, F BarocchiAbstract:Extending a previous study of the Velocity Autocorrelation Function (VAF) in a simulated Lennard-Jones fluid to cover higher-density and lower-temperature states, we show that the recently demonstrated multiexponential expansion allows for a full account and understanding of the dynamical processes encompassed by a fundamental quantity as the VAF. In particular, besides obtaining evidence of a persisting long-time tail, we assign specific and unambiguous physical meanings to groups of exponential modes related to the longitudinal and transverse collective dynamics, respectively. We have made this possible by consistently introducing the interpretation of the VAF frequency spectrum as a global density of states in fluids, generalizing a solid-state concept, and by giving to specific spectral components, obtained via the VAF exponential expansion, the corresponding meaning of partial densities of states relative to specific dynamical processes. The clear identification of a high-frequency oscillation of the VAF with the near-top excitation frequency in the dispersion curve of acoustic waves is a neat example of the power of the method. As for the transverse mode contribution, its analysis turns out to be particularly important, because the multiexponential expansion reveals a transition marking the onset of propagating excitations when the density is increased above a threshold value. While this finding agrees with recent literature debating the issue of dynamical crossover boundaries, such as the one identified with the Frenkel line, we can add detailed information on the modes involved in this specific process in the domains of both time and frequency. This will help obtain a still missing full account of transverse dynamics, in its nonpropagating and propagating aspects which are linked by dynamical transitions depending on both the thermodynamic states and the excitation wavevectors.
Masaharu Isobe - One of the best experts on this subject based on the ideXlab platform.
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long time tail of the Velocity Autocorrelation Function in a two dimensional moderately dense hard disk fluid
Bulletin of the American Physical Society, 2008Co-Authors: Masaharu IsobeAbstract:Alder and Wainwright discovered the slow power decay $\ensuremath{\sim}{t}^{\ensuremath{-}d/2}$ ($d$ is dimension) of the Velocity Autocorrelation Function in moderately dense hard-sphere fluids using the event-driven molecular dynamics simulations. In the two-dimensional (2D) case, the diffusion coefficient derived using the time correlation expression in linear response theory shows logarithmic divergence, which is called the ``2D long-time-tail problem.'' We reexamined this problem to perform a large-scale, long-time simulation with $1\ifmmode\times\else\texttimes\fi{}{10}^{6}$ hard disks using a modern efficient algorithm and found that the decay of the long tail in moderately dense fluids is slightly faster than the power decay $(\ensuremath{\sim}1/t)$. We also compared our numerical data with the prediction of the self-consistent mode-coupling theory in the long-time limit $[\ensuremath{\sim}1/(t\sqrt{\text{ln}\text{ }t})]$.
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long time tail of the Velocity Autocorrelation Function in a two dimensional moderately dense hard disk fluid
Physical Review E, 2008Co-Authors: Masaharu IsobeAbstract:Alder and Wainwright discovered the slow power decay ~t(-d/2) (d is dimension) of the Velocity Autocorrelation Function in moderately dense hard-sphere fluids using the event-driven molecular dynamics simulations. In the two-dimensional (2D) case, the diffusion coefficient derived using the time correlation expression in linear response theory shows logarithmic divergence, which is called the "2D long-time-tail problem." We reexamined this problem to perform a large-scale, long-time simulation with 1x10(6) hard disks using a modern efficient algorithm and found that the decay of the long tail in moderately dense fluids is slightly faster than the power decay (~1/t) . We also compared our numerical data with the prediction of the self-consistent mode-coupling theory in the long-time limit [~1/(t sqrt[ln t])] .
F Barocchi - One of the best experts on this subject based on the ideXlab platform.
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Density and time scaling effects on the Velocity Autocorrelation Function of quantum and classical dense fluid para-hydrogen.
The Journal of chemical physics, 2019Co-Authors: Stefano Bellissima, M Neumann, U Bafile, F Barocchi, Daniele Colognesi, E GuariniAbstract:We report the results of a ring polymer molecular dynamics study of the Kubo Velocity Autocorrelation Function of a quantum fluid as para-hydrogen aimed at the comparison with its classical counterpart. Quite different density conditions were considered for both the classical and quantum cases, in order to compare the two systems before and after the dynamical crossover typically undergone by the Velocity Autocorrelation Function (VAF) of fluids at densities around the triple point, where the shape of the Function changes from a monotonic to an oscillatory behavior with a negative minimum. A detailed study of the phase diagram of classical para-hydrogen was necessary for a reasonable choice of the classical states to be taken into consideration, in the spirit of the classical principle of corresponding states. The shape of the quantum and classical VAF was thoroughly analyzed, exhibiting at all studied densities clear differences that might be taken as evidence of quantum effects. We show that these differences are substantially reduced by applying a state-dependent time scaling with respect to a reference time identified with the inverse of the collision rate. An even better coincidence in shape is found by comparing the two systems at slightly non-corresponding reduced densities, suggesting that the quantum system behaves almost like the classical one, but at systematically less dense reduced states of the latter. We also find an unexpected and quite interesting density trend of the collision rate of both classical and quantum para-hydrogen, which accounts for the effectiveness of the scaling throughout the explored density range. The mean kinetic energy and the diffusion coefficients are also discussed in some detail.We report the results of a ring polymer molecular dynamics study of the Kubo Velocity Autocorrelation Function of a quantum fluid as para-hydrogen aimed at the comparison with its classical counterpart. Quite different density conditions were considered for both the classical and quantum cases, in order to compare the two systems before and after the dynamical crossover typically undergone by the Velocity Autocorrelation Function (VAF) of fluids at densities around the triple point, where the shape of the Function changes from a monotonic to an oscillatory behavior with a negative minimum. A detailed study of the phase diagram of classical para-hydrogen was necessary for a reasonable choice of the classical states to be taken into consideration, in the spirit of the classical principle of corresponding states. The shape of the quantum and classical VAF was thoroughly analyzed, exhibiting at all studied densities clear differences that might be taken as evidence of quantum effects. We show that these diffe...
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density and time scaling effects on the Velocity Autocorrelation Function of quantum and classical dense fluid para hydrogen
Journal of Chemical Physics, 2019Co-Authors: Stefano Bellissima, U Bafile, F Barocchi, Daniele Colognesi, M Neumann, E GuariniAbstract:We report the results of a ring polymer molecular dynamics study of the Kubo Velocity Autocorrelation Function of a quantum fluid as para-hydrogen aimed at the comparison with its classical counterpart. Quite different density conditions were considered for both the classical and quantum cases, in order to compare the two systems before and after the dynamical crossover typically undergone by the Velocity Autocorrelation Function (VAF) of fluids at densities around the triple point, where the shape of the Function changes from a monotonic to an oscillatory behavior with a negative minimum. A detailed study of the phase diagram of classical para-hydrogen was necessary for a reasonable choice of the classical states to be taken into consideration, in the spirit of the classical principle of corresponding states. The shape of the quantum and classical VAF was thoroughly analyzed, exhibiting at all studied densities clear differences that might be taken as evidence of quantum effects. We show that these differences are substantially reduced by applying a state-dependent time scaling with respect to a reference time identified with the inverse of the collision rate. An even better coincidence in shape is found by comparing the two systems at slightly non-corresponding reduced densities, suggesting that the quantum system behaves almost like the classical one, but at systematically less dense reduced states of the latter. We also find an unexpected and quite interesting density trend of the collision rate of both classical and quantum para-hydrogen, which accounts for the effectiveness of the scaling throughout the explored density range. The mean kinetic energy and the diffusion coefficients are also discussed in some detail.
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density of states and dynamical crossover in a dense fluid revealed by exponential mode analysis of the Velocity Autocorrelation Function
Physical Review E, 2017Co-Authors: Stefano Bellissima, M Neumann, E Guarini, U Bafile, F BarocchiAbstract:Extending a preceding study of the Velocity Autocorrelation Function (VAF) in a simulated Lennard-Jones fluid [Phys. Rev. E 92, 042166 (2015)PLEEE81539-375510.1103/PhysRevE.92.042166] to cover higher-density and lower-temperature states, we show that the recently demonstrated multiexponential expansion method allows for a full account and understanding of the basic dynamical processes encompassed by a fundamental quantity as the VAF. In particular, besides obtaining evidence of a persisting long-time tail, we assign specific and unambiguous physical meanings to groups of exponential modes related to the longitudinal and transverse collective dynamics, respectively. We have made this possible by consistently introducing the interpretation of the VAF frequency spectrum as a global density of states in fluids, generalizing a solid-state concept, and by giving to specific spectral components, obtained through the VAF exponential expansion, the corresponding meaning of partial densities of states relative to specific dynamical processes. The clear identification of a high-frequency oscillation of the VAF with the near-top excitation frequency in the dispersion curve of acoustic waves is a neat example of the power of the method. As for the transverse mode contribution, its analysis turns out to be particularly important, because the multiexponential expansion reveals a transition marking the onset of propagating excitations when the density is increased beyond a threshold value. While this finding agrees with the recent literature debating the issue of dynamical crossover boundaries, such as the one identified with the Frenkel line, we can add detailed information on the modes involved in this specific process in the domains of both time and frequency. This will help obtain a still missing full account of transverse dynamics, in both its nonpropagating and propagating aspects which are linked through dynamical transitions depending on both the thermodynamic states and the excitation wave vectors.
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density of states and dynamical crossover in a dense fluid revealed by exponential mode analysis of the Velocity Autocorrelation Function
arXiv: Statistical Mechanics, 2016Co-Authors: Stefano Bellissima, M Neumann, U Bafile, E Guarini, F BarocchiAbstract:Extending a previous study of the Velocity Autocorrelation Function (VAF) in a simulated Lennard-Jones fluid to cover higher-density and lower-temperature states, we show that the recently demonstrated multiexponential expansion allows for a full account and understanding of the dynamical processes encompassed by a fundamental quantity as the VAF. In particular, besides obtaining evidence of a persisting long-time tail, we assign specific and unambiguous physical meanings to groups of exponential modes related to the longitudinal and transverse collective dynamics, respectively. We have made this possible by consistently introducing the interpretation of the VAF frequency spectrum as a global density of states in fluids, generalizing a solid-state concept, and by giving to specific spectral components, obtained via the VAF exponential expansion, the corresponding meaning of partial densities of states relative to specific dynamical processes. The clear identification of a high-frequency oscillation of the VAF with the near-top excitation frequency in the dispersion curve of acoustic waves is a neat example of the power of the method. As for the transverse mode contribution, its analysis turns out to be particularly important, because the multiexponential expansion reveals a transition marking the onset of propagating excitations when the density is increased above a threshold value. While this finding agrees with recent literature debating the issue of dynamical crossover boundaries, such as the one identified with the Frenkel line, we can add detailed information on the modes involved in this specific process in the domains of both time and frequency. This will help obtain a still missing full account of transverse dynamics, in its nonpropagating and propagating aspects which are linked by dynamical transitions depending on both the thermodynamic states and the excitation wavevectors.
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time dependence of the Velocity Autocorrelation Function of a fluid an eigenmode analysis of dynamical processes
Physical Review E, 2015Co-Authors: Stefano Bellissima, U Bafile, M Neumann, E Guarini, F BarocchiAbstract:The Velocity Autocorrelation Function (VAF), a key quantity in the atomic-scale dynamics of fluids, has been the first paradigmatic example of a long-time tail phenomenon, and much work has been devoted to detecting such long-lasting correlations and understanding their nature. There is, however, much more to the VAF than simply the evidence of this long-time dynamics. A unified description of the VAF from very short to long times, and of the way it changes with varying density, is still missing. Here we show that an approach based on very general principles makes such a study possible and opens the way to a detailed quantitative characterization of the dynamical processes involved at all time scales. From the analysis of molecular dynamics simulations for a slightly supercritical Lennard-Jones fluid at various densities, we are able to evidence the presence of distinct fast and slow decay channels for the Velocity correlation on the time scale set by the collision rate. The density evolution of these decay processes is also highlighted. The method presented here is very general, and its application to the VAF can be considered as an important example.
U Bafile - One of the best experts on this subject based on the ideXlab platform.
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density dependence of the dynamical processes governing the Velocity Autocorrelation Function of a quantum fluid
Physical Review E, 2019Co-Authors: E Guarini, Stefano Bellissima, Daniele Colognesi, M Neumann, U BafileAbstract:We present an exponential mode analysis of the dynamical processes determining the time behavior of the Kubo Velocity Autocorrelation Function (KVAF) of fluid para-${\mathrm{H}}_{2}$, as obtained by ring polymer molecular dynamics simulations at various fluid densities. The mechanisms contributing to the decay of the KVAF are thoroughly characterized at a slightly supercritical temperature, in a density interval ranging from the critical point to the fluid-solid transition. We show that the quantum nature of the system does not influence the specific phenomena and decay channels through which a loss in Velocity correlation takes place, since these are the same as found in classical fluids. Similarly, a dynamical crossover is observed with increasing density, signaling the onset of a transverse-like dynamics like in classical systems. We also investigate the effect of density on the processes contributing to the most relevant property of a quantum fluid, namely, the large values of the total and zero-point kinetic energy arising through the Heisenberg uncertainty principle.
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Density and time scaling effects on the Velocity Autocorrelation Function of quantum and classical dense fluid para-hydrogen.
The Journal of chemical physics, 2019Co-Authors: Stefano Bellissima, M Neumann, U Bafile, F Barocchi, Daniele Colognesi, E GuariniAbstract:We report the results of a ring polymer molecular dynamics study of the Kubo Velocity Autocorrelation Function of a quantum fluid as para-hydrogen aimed at the comparison with its classical counterpart. Quite different density conditions were considered for both the classical and quantum cases, in order to compare the two systems before and after the dynamical crossover typically undergone by the Velocity Autocorrelation Function (VAF) of fluids at densities around the triple point, where the shape of the Function changes from a monotonic to an oscillatory behavior with a negative minimum. A detailed study of the phase diagram of classical para-hydrogen was necessary for a reasonable choice of the classical states to be taken into consideration, in the spirit of the classical principle of corresponding states. The shape of the quantum and classical VAF was thoroughly analyzed, exhibiting at all studied densities clear differences that might be taken as evidence of quantum effects. We show that these differences are substantially reduced by applying a state-dependent time scaling with respect to a reference time identified with the inverse of the collision rate. An even better coincidence in shape is found by comparing the two systems at slightly non-corresponding reduced densities, suggesting that the quantum system behaves almost like the classical one, but at systematically less dense reduced states of the latter. We also find an unexpected and quite interesting density trend of the collision rate of both classical and quantum para-hydrogen, which accounts for the effectiveness of the scaling throughout the explored density range. The mean kinetic energy and the diffusion coefficients are also discussed in some detail.We report the results of a ring polymer molecular dynamics study of the Kubo Velocity Autocorrelation Function of a quantum fluid as para-hydrogen aimed at the comparison with its classical counterpart. Quite different density conditions were considered for both the classical and quantum cases, in order to compare the two systems before and after the dynamical crossover typically undergone by the Velocity Autocorrelation Function (VAF) of fluids at densities around the triple point, where the shape of the Function changes from a monotonic to an oscillatory behavior with a negative minimum. A detailed study of the phase diagram of classical para-hydrogen was necessary for a reasonable choice of the classical states to be taken into consideration, in the spirit of the classical principle of corresponding states. The shape of the quantum and classical VAF was thoroughly analyzed, exhibiting at all studied densities clear differences that might be taken as evidence of quantum effects. We show that these diffe...
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density and time scaling effects on the Velocity Autocorrelation Function of quantum and classical dense fluid para hydrogen
Journal of Chemical Physics, 2019Co-Authors: Stefano Bellissima, U Bafile, F Barocchi, Daniele Colognesi, M Neumann, E GuariniAbstract:We report the results of a ring polymer molecular dynamics study of the Kubo Velocity Autocorrelation Function of a quantum fluid as para-hydrogen aimed at the comparison with its classical counterpart. Quite different density conditions were considered for both the classical and quantum cases, in order to compare the two systems before and after the dynamical crossover typically undergone by the Velocity Autocorrelation Function (VAF) of fluids at densities around the triple point, where the shape of the Function changes from a monotonic to an oscillatory behavior with a negative minimum. A detailed study of the phase diagram of classical para-hydrogen was necessary for a reasonable choice of the classical states to be taken into consideration, in the spirit of the classical principle of corresponding states. The shape of the quantum and classical VAF was thoroughly analyzed, exhibiting at all studied densities clear differences that might be taken as evidence of quantum effects. We show that these differences are substantially reduced by applying a state-dependent time scaling with respect to a reference time identified with the inverse of the collision rate. An even better coincidence in shape is found by comparing the two systems at slightly non-corresponding reduced densities, suggesting that the quantum system behaves almost like the classical one, but at systematically less dense reduced states of the latter. We also find an unexpected and quite interesting density trend of the collision rate of both classical and quantum para-hydrogen, which accounts for the effectiveness of the scaling throughout the explored density range. The mean kinetic energy and the diffusion coefficients are also discussed in some detail.
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density of states and dynamical crossover in a dense fluid revealed by exponential mode analysis of the Velocity Autocorrelation Function
Physical Review E, 2017Co-Authors: Stefano Bellissima, M Neumann, E Guarini, U Bafile, F BarocchiAbstract:Extending a preceding study of the Velocity Autocorrelation Function (VAF) in a simulated Lennard-Jones fluid [Phys. Rev. E 92, 042166 (2015)PLEEE81539-375510.1103/PhysRevE.92.042166] to cover higher-density and lower-temperature states, we show that the recently demonstrated multiexponential expansion method allows for a full account and understanding of the basic dynamical processes encompassed by a fundamental quantity as the VAF. In particular, besides obtaining evidence of a persisting long-time tail, we assign specific and unambiguous physical meanings to groups of exponential modes related to the longitudinal and transverse collective dynamics, respectively. We have made this possible by consistently introducing the interpretation of the VAF frequency spectrum as a global density of states in fluids, generalizing a solid-state concept, and by giving to specific spectral components, obtained through the VAF exponential expansion, the corresponding meaning of partial densities of states relative to specific dynamical processes. The clear identification of a high-frequency oscillation of the VAF with the near-top excitation frequency in the dispersion curve of acoustic waves is a neat example of the power of the method. As for the transverse mode contribution, its analysis turns out to be particularly important, because the multiexponential expansion reveals a transition marking the onset of propagating excitations when the density is increased beyond a threshold value. While this finding agrees with the recent literature debating the issue of dynamical crossover boundaries, such as the one identified with the Frenkel line, we can add detailed information on the modes involved in this specific process in the domains of both time and frequency. This will help obtain a still missing full account of transverse dynamics, in both its nonpropagating and propagating aspects which are linked through dynamical transitions depending on both the thermodynamic states and the excitation wave vectors.
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density of states and dynamical crossover in a dense fluid revealed by exponential mode analysis of the Velocity Autocorrelation Function
arXiv: Statistical Mechanics, 2016Co-Authors: Stefano Bellissima, M Neumann, U Bafile, E Guarini, F BarocchiAbstract:Extending a previous study of the Velocity Autocorrelation Function (VAF) in a simulated Lennard-Jones fluid to cover higher-density and lower-temperature states, we show that the recently demonstrated multiexponential expansion allows for a full account and understanding of the dynamical processes encompassed by a fundamental quantity as the VAF. In particular, besides obtaining evidence of a persisting long-time tail, we assign specific and unambiguous physical meanings to groups of exponential modes related to the longitudinal and transverse collective dynamics, respectively. We have made this possible by consistently introducing the interpretation of the VAF frequency spectrum as a global density of states in fluids, generalizing a solid-state concept, and by giving to specific spectral components, obtained via the VAF exponential expansion, the corresponding meaning of partial densities of states relative to specific dynamical processes. The clear identification of a high-frequency oscillation of the VAF with the near-top excitation frequency in the dispersion curve of acoustic waves is a neat example of the power of the method. As for the transverse mode contribution, its analysis turns out to be particularly important, because the multiexponential expansion reveals a transition marking the onset of propagating excitations when the density is increased above a threshold value. While this finding agrees with recent literature debating the issue of dynamical crossover boundaries, such as the one identified with the Frenkel line, we can add detailed information on the modes involved in this specific process in the domains of both time and frequency. This will help obtain a still missing full account of transverse dynamics, in its nonpropagating and propagating aspects which are linked by dynamical transitions depending on both the thermodynamic states and the excitation wavevectors.
E Guarini - One of the best experts on this subject based on the ideXlab platform.
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density dependence of the dynamical processes governing the Velocity Autocorrelation Function of a quantum fluid
Physical Review E, 2019Co-Authors: E Guarini, Stefano Bellissima, Daniele Colognesi, M Neumann, U BafileAbstract:We present an exponential mode analysis of the dynamical processes determining the time behavior of the Kubo Velocity Autocorrelation Function (KVAF) of fluid para-${\mathrm{H}}_{2}$, as obtained by ring polymer molecular dynamics simulations at various fluid densities. The mechanisms contributing to the decay of the KVAF are thoroughly characterized at a slightly supercritical temperature, in a density interval ranging from the critical point to the fluid-solid transition. We show that the quantum nature of the system does not influence the specific phenomena and decay channels through which a loss in Velocity correlation takes place, since these are the same as found in classical fluids. Similarly, a dynamical crossover is observed with increasing density, signaling the onset of a transverse-like dynamics like in classical systems. We also investigate the effect of density on the processes contributing to the most relevant property of a quantum fluid, namely, the large values of the total and zero-point kinetic energy arising through the Heisenberg uncertainty principle.
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density and time scaling effects on the Velocity Autocorrelation Function of quantum and classical dense fluid para hydrogen
Journal of Chemical Physics, 2019Co-Authors: Stefano Bellissima, U Bafile, F Barocchi, Daniele Colognesi, M Neumann, E GuariniAbstract:We report the results of a ring polymer molecular dynamics study of the Kubo Velocity Autocorrelation Function of a quantum fluid as para-hydrogen aimed at the comparison with its classical counterpart. Quite different density conditions were considered for both the classical and quantum cases, in order to compare the two systems before and after the dynamical crossover typically undergone by the Velocity Autocorrelation Function (VAF) of fluids at densities around the triple point, where the shape of the Function changes from a monotonic to an oscillatory behavior with a negative minimum. A detailed study of the phase diagram of classical para-hydrogen was necessary for a reasonable choice of the classical states to be taken into consideration, in the spirit of the classical principle of corresponding states. The shape of the quantum and classical VAF was thoroughly analyzed, exhibiting at all studied densities clear differences that might be taken as evidence of quantum effects. We show that these differences are substantially reduced by applying a state-dependent time scaling with respect to a reference time identified with the inverse of the collision rate. An even better coincidence in shape is found by comparing the two systems at slightly non-corresponding reduced densities, suggesting that the quantum system behaves almost like the classical one, but at systematically less dense reduced states of the latter. We also find an unexpected and quite interesting density trend of the collision rate of both classical and quantum para-hydrogen, which accounts for the effectiveness of the scaling throughout the explored density range. The mean kinetic energy and the diffusion coefficients are also discussed in some detail.
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density of states and dynamical crossover in a dense fluid revealed by exponential mode analysis of the Velocity Autocorrelation Function
arXiv: Statistical Mechanics, 2016Co-Authors: Stefano Bellissima, M Neumann, U Bafile, E Guarini, F BarocchiAbstract:Extending a previous study of the Velocity Autocorrelation Function (VAF) in a simulated Lennard-Jones fluid to cover higher-density and lower-temperature states, we show that the recently demonstrated multiexponential expansion allows for a full account and understanding of the dynamical processes encompassed by a fundamental quantity as the VAF. In particular, besides obtaining evidence of a persisting long-time tail, we assign specific and unambiguous physical meanings to groups of exponential modes related to the longitudinal and transverse collective dynamics, respectively. We have made this possible by consistently introducing the interpretation of the VAF frequency spectrum as a global density of states in fluids, generalizing a solid-state concept, and by giving to specific spectral components, obtained via the VAF exponential expansion, the corresponding meaning of partial densities of states relative to specific dynamical processes. The clear identification of a high-frequency oscillation of the VAF with the near-top excitation frequency in the dispersion curve of acoustic waves is a neat example of the power of the method. As for the transverse mode contribution, its analysis turns out to be particularly important, because the multiexponential expansion reveals a transition marking the onset of propagating excitations when the density is increased above a threshold value. While this finding agrees with recent literature debating the issue of dynamical crossover boundaries, such as the one identified with the Frenkel line, we can add detailed information on the modes involved in this specific process in the domains of both time and frequency. This will help obtain a still missing full account of transverse dynamics, in its nonpropagating and propagating aspects which are linked by dynamical transitions depending on both the thermodynamic states and the excitation wavevectors.
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time dependence of the Velocity Autocorrelation Function of a fluid an eigenmode analysis of dynamical processes
Physical Review E, 2015Co-Authors: Stefano Bellissima, U Bafile, M Neumann, E Guarini, F BarocchiAbstract:The Velocity Autocorrelation Function (VAF), a key quantity in the atomic-scale dynamics of fluids, has been the first paradigmatic example of a long-time tail phenomenon, and much work has been devoted to detecting such long-lasting correlations and understanding their nature. There is, however, much more to the VAF than simply the evidence of this long-time dynamics. A unified description of the VAF from very short to long times, and of the way it changes with varying density, is still missing. Here we show that an approach based on very general principles makes such a study possible and opens the way to a detailed quantitative characterization of the dynamical processes involved at all time scales. From the analysis of molecular dynamics simulations for a slightly supercritical Lennard-Jones fluid at various densities, we are able to evidence the presence of distinct fast and slow decay channels for the Velocity correlation on the time scale set by the collision rate. The density evolution of these decay processes is also highlighted. The method presented here is very general, and its application to the VAF can be considered as an important example.