Collective dynamics and molecular interactions in liquid<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>CO</mml:mtext></mml:mrow><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>by inelastic neutron scattering and computer simulations

Sampoli, M.; Bafile, Ubaldo; Guarini, Eleonora; Barocchi, F.

doi:10.1103/physrevb.79.214203

Cited by 14 publications

(16 citation statements)

References 65 publications

(61 reference statements)

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“…This phenomenon is one of the causes of the bending upwards of the dispersion curve, whose overall shape is however determined by a number of facts [35]. In particular, the growth of the damping z s with Q tends instead to reduce the excitation frequency up to the point where the acoustic oscillations are brought into the overdamping condition causing the above mentioned propagation gap [50,51].…”

Section: Discussionmentioning

confidence: 99%

Density of states and dynamical crossover in a dense fluid revealed by exponential mode analysis of the velocity autocorrelation function

et al. 2017

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Extending a preceding study of the velocity autocorrelation function (VAF) in a simulated Lennard-Jones fluid [Phys. Rev. E 92, 042166 (2015)] to cover higher-density and lower-temperature states, we show that the recently demonstrated multi-exponential 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 multi-exponential 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 wavevectors.

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Section: Discussionmentioning

confidence: 99%

Density of states and dynamical crossover in a dense fluid revealed by exponential mode analysis of the velocity autocorrelation function

et al. 2017

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“…Therefore, we analyze spectra obtained by molecular dynamics simulations, which usually present the further advantage of being calculated in broad frequency ranges. However, the compatibility of simulations with available experimental data has been demonstrated [21,22] as a preliminary, mandatory task to be carried out before confidently resorting to simulated data.…”

Section: Applicationsmentioning

confidence: 97%

“…CO 2 can be considered as a classical fluid, so that the expansion in Lorentzian functions can be applied directly to the structure factor, which for a homogeneous and isotropic fluid depends only on the magnitude k of the wave vector k. In Ref. [22] it was shown that the spectrum is well described if the continued fraction is truncated at the level of the second-order memory function and a viscoelastic modeling for the latter is assumed, which is known to give rise to a four-Lorentzian spectrum [17]. In the whole k range, with the exception of an interval around the position of the main peak of the static structure factor S(k), two of the four lines are characterized by complex eigenfrequencies and correspond to the excitation of acoustic modes.…”

Section: B the Dynamic Structure Factor Of Liquid Comentioning

confidence: 99%

“…[22] where a neutron scattering experimental validation of molecular dynamics simulations of liquid carbon dioxide allowed for a detailed analysis of the partial carbon-carbon dynamic structure factor. CO 2 can be considered as a classical fluid, so that the expansion in Lorentzian functions can be applied directly to the structure factor, which for a homogeneous and isotropic fluid depends only on the magnitude k of the wave vector k. In Ref.…”

Section: B the Dynamic Structure Factor Of Liquid Comentioning

confidence: 99%

“…Here, as examples of spectra which can be represented by a finite number of Lorentzian shapes, we present the cases of the velocity autocorrelation function of liquid parahydrogen [21] and of the center-of-mass dynamic structure factor of the molecular liquid CO 2 , representing the collective translational dynamics [22]. The first example refers to a system displaying a non-negligible quantum behavior, while the second pertains to an essentially classical case.…”

Section: Introductionmentioning

confidence: 99%

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Expansion in Lorentzian functions of spectra of quantum autocorrelations

Barocchi

Bafile

2013

Phys. Rev. E

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We show that in a quantum mechanical many-body system of Boltzmann particles having space inversion symmetry the spectrum of the autocorrelation function of a local observable can always be given, similarly to the classical case [Phys. Rev. E 85, 022102 (2012)], in terms of a series of Lorentzian functions multiplied by the proper quantum detailed balance factor. This is done by transforming the continued fraction representation, which is derived via recurrent relations and without the use of the generalized Langevin equation hierarchy, into a series expansion. In this way characteristic frequencies can be defined, also in quantum mechanics, which refer to the particular autocorrelation. These are the frequencies of the eigenmodes of the relaxation function connected to the observable. We also show that in practical cases of interest in experimental spectroscopy, and particularly in inelastic neutron and x-ray scattering, the use of a finite number of Lorentzian shapes for an approximate description of the data is related to a reduction of the number of the relevant dynamical variables taken into account, equivalent to the lowering of the dimensionality of the orthogonalized space onto which the dynamic of the system is projected. Examples of application are given for the spectrum of the velocity autocorrelation function of liquid parahydrogen, calculated with a quantum simulation algorithm (path-integral centroid molecular dynamics), and for the molecular center-of mass dynamic structure factor in liquid carbon dioxide as computed by means of classical molecular dynamics simulation.

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Switching off hydrogen-bond-driven excitation modes in liquid methanol

Bellissima

González

Bafile

et al. 2017

Sci Rep

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Hydrogen bonding plays an essential role on intermolecular forces, and consequently on the thermodynamics of materials defined by this elusive bonding character. It determines the property of a vital liquid as water as well as many processes crucial for life. The longstanding controversy on the nature of the hydrogen bond (HB) can be settled by looking at the effect of a vanishing HB interaction on the microscopic properties of a given hydrogen-bonded fluid. This task suits the capabilities of computer simulations techniques, which allow to easily switch off HB interactions. We then use molecular dynamics to study the microscopic properties of methanol, a prototypical HB liquid. Fundamental aspects of the dynamics of methanol at room temperature were contextualised only very recently and its rich dynamics was found to have striking analogies with that of water. The lower temperature (200 K) considered in the present study led us to observe that the molecular centre-of-mass dynamics is dominated by four modes. Most importantly, the computational ability to switch on and off hydrogen bonds permitted us to identify which, among these modes, have a pure HB-origin. This clarifies the role of hydrogen bonds in liquid dynamics, disclosing new research opportunities and unexplored interpretation schemes.

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Collective dynamics and molecular interactions in liquidCO2by inelastic neutron scattering and computer simulations

Cited by 14 publications

References 65 publications

Density of states and dynamical crossover in a dense fluid revealed by exponential mode analysis of the velocity autocorrelation function

Density of states and dynamical crossover in a dense fluid revealed by exponential mode analysis of the velocity autocorrelation function

Expansion in Lorentzian functions of spectra of quantum autocorrelations

Switching off hydrogen-bond-driven excitation modes in liquid methanol

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