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There is indeed a finite-size issue in computer simulations of the correlation decay in a quasi-one-dimensional system of hard disks. The correlation function obtained from the exact canonical NVT partition function shows a decay which is much slower and more complex than the exponential one, presented in the Comment.

The simplicity of a quasi-one-dimensional system of hard disks enables us to get a much deeper and more quantitative insight into the solid-to-fluid transformation and identify it as caging-uncaging transition. Both computer simulation data and theoretical results show that density decrease induces progressively larger number of uncaged disk pairs which exchange their transverse positions through windows in the initial crystalline zigzag array. At low densities windows dominate and disks are moving independently (pairs “dissociate”). Along with the simulation data on the transversal excitation modes and pair correlation function this shows that the solid-to-liquid transformation is a continuous Kosterlitz-Thouless-type transition.

In May 2020 Ihor Mryglod, who is one of the most renowned physicists in Ukraine, full member of the National Academy of Sciences of Ukraine (NASU) and Director of the Institute for Condensed Matter Physics of NASU, is turning 60.

Non-monotonous changes in velocity autocorrelations across the transformation from molecular to atomic fluid in hydrogen under pressure are studied by ab initio molecular dynamics simulations at the temperature 2500 K. We report diffusion coefficients in a wide range of densities from purely molecular fluid up to metallic atomic fluid phase. An analysis of contributions to the velocity autocorrelation functions from the motion of molecular centers-of-mass, rotational and intramolecular vibrational modes is performed, and a crossover in the vibrational density of intramolecular modes across the transition is discussed.

High-resolution inelastic x-ray scattering measurements were carried out on molten NaI near the melting point at 680°C at SPring-8. Small and damped indications of longitudinal optic excitation modes were observed on the tails of the longitudinal acoustic modes at small momentum transfers, Q∼5 nm^{-1}. The measured spectra are in good agreement, in both frequency and linewidth, with ab initio molecular dynamics (MD) simulations but not classical MD simulations. The observation of these modes at small Q and a good agreement with the simulation permits clear identification of these as collective optic modes with well defined phasing between different ionic motions.

This Special issue of the "Condensed Matter Physics" journal is dedicated to Professor Giancarlo Ruocco on the occasion of his 60 birthday. Giancarlo Ruocco is widely known as one of the pioneers in the development of the meV resolution Inelastic X-ray Scattering (IXS), a novel and yet state-of-the-art technique for studying atomistic dynamics in condensed and soft matter. The IXS opened new possibilities in understanding the microscopic nature of relaxation processes in liquids, glasses, complex biomolecular systems and polymers. His strong background in theory allowed Giancarlo Ruocco to combine experimental findings with theoretical analysis of IXS data, that led to fundamental results for dynamic processes in various condensed and soft matter systems.

A simple ansatz for the study of velocity autocorrelation functions in fluids at different timescales is proposed. The ansatz is based on an effective summation of the infinite continued fraction at a reasonable assumption about convergence of relaxation times of the higher order memory functions, which have a purely kinetic origin. The VAFs obtained within our approach are compared with the results of the Markovian approximation for memory kernels. It is shown that although in the "overdamped" regime both approaches agree to a large extent at the initial and intermediate times of the system evolution, our formalism yields power law relaxation of the VAFs which is not observed at the description with a finite number of the collective modes. Explicit expressions for the transition times from kinetic to hydrodynamic regimes are obtained from the analysis of the singularities of spectral functions in the complex frequency plane.

It is our great pleasure to present a Special issue of Condensed Matter Physics in honour of the renowned scientist and our former scientific advisor Professor A.D.J. "Tony" Haymet, who turned 60 this February. An internationally-known physical chemist and scientific manager with over 170 scientific publications and numerous op-ed pieces, Tony Haymet served from 2006 until 2012 as the Director of the Scripps Institution of Oceanography, Vice Chancellor for Marine Sciences and Dean of the Graduate School of Marine Sciences at the University of California at San Diego (UCSD), where he is currently a Distinguished Professor.

The collective dynamics in liquid water is an active research topic experimentally, theoretically and via simulations. Here, ab initio molecular dynamics simulations are reported in heavy and ordinary water at temperature 323.15 K, or 50^{o}C. The simulations in heavy water were performed both with and without dispersion corrections. We found that the dispersion correction (DFT-D3) changes the relaxation of density-density time correlation functions from a slow, typical of a supercooled state, to exponential decay behaviour of regular liquids. This implies an essential reduction of the melting point of ice in simulations with DFT-D3. Analysis of longitudinal (L) and transverse (T) current spectral functions allowed us to estimate the dispersions of acoustic and optic collective excitations and to observe the L-T mixing effect. The dispersion correction shifts the L and T optic (O) modes to lower frequencies and provides by almost thirty per cent smaller gap between the longest-wavelength LO and TO excitations, which can be a consequence of a larger effective high-frequency dielectric permittivity in simulations with dispersion corrections. Simulation in ordinary water with the dispersion correction results in frequencies of optic excitations higher than in D_{2}O, and in a long-wavelength LO-TO gap of 24 ps^{-1} (127 cm^{-1}).

Ice-water, water-vapor interfaces and ice surface are studied by molecular dynamics simulations with the SPC/E model of water molecules having the purpose to estimate the profiles of electrostatic potential across the interfaces. We have proposed a methodology for calculating the profiles of electrostatic potential based on a trial particle, which showed good agreement for the case of electrostatic potential profile of the water-vapor interface of TIP4P model calculated in another way. The measured profile of electrostatic potential for the pure ice-water interface decreases towards the liquid bulk region, which is in agreement with simulations of preferential direction of motion of Li^{+} and F^{-} solute ions at the liquid side of the ice-water interface. These results are discussed in connection with the Workman-Reynolds effect.

We study autocorrelation functions of energy, heat and entropy densities obtained by molecular dynamics simulations of supercritical Ar and compare them with the predictions of the hydrodynamic theory. It is shown that the predicted by the hydrodynamic theory single-exponential shape of the entropy density autocorrelation functions is perfectly reproduced for small wave numbers by the molecular dynamics simulations and permits the calculation of the wavenumber-dependent specific heat at constant pressure. The estimated wavenumber-dependent specific heats at constant volume and pressure, Cv(k) and Cp(k), are shown to be in the long-wavelength limit in good agreement with the macroscopic experimental values of Cv and Cp for the studied thermodynamic points of supercritical Ar.

The effect of grape powders on the rheological properties of dough and the indicators of butter biscuits quality have been investigated. Butter biscuits were cooked with the addition of powdered grape seeds and grape skins from the grape pomace obtained in the southern regions of Ukraine. The grape powders were added to the dough in an amount of 15.0% of the weight of the wheat flour. It has been shown that the presence of powdered grape skins increases the dough’s effective viscosity by 3.9 times, of powdered grape seeds – by 5.4 times, as compared to the reference sample, and it makes the dough more resistant to destruction. Adding powdered grape skins increases the plastic viscosity of the samples by 31.4%, powdered grape seeds – by 38.1%. The higher plastic properties of the dough allow the biscuits to hold their shape better and to retain the pattern on their surface. With grape powders added, the adhesive strength of the dough decreases. When powdered grape skins are added, the adhesion strength decreases by 23.0 %, and if we add powdered grape seeds, by 33.0%. It facilitates the work of moulding equipment and reduces the production losses of the dough during processing and moulding. The values of physicochemical parameters of the quality of butter biscuits – the specific volume and the wetting ability – slightly increase as compared to the reference sample. Products hold their shape better, and do not become runny during baking, because of a higher degree of form stability. Organoleptic indicators of the quality of butter biscuits with experimental additives are becoming better. Biscuits acquire a pleasant nutty taste after adding powdered grape seeds, and the taste and flavour of prunes and raisins after adding powdered grape skins. Butter biscuits take a deep chocolate colour or chocolate with a violet tint.

We present ab initio simulations of a large system of 2400 particles of molten NaCl to investigate the behavior of collective mode dispersion beyond the hydrodynamic regime. In particular, we aim to explain the unusually strong increase in the apparent speed of sound with wave number, which significantly exceeds the typical positive sound dispersion of 10%-25% observed in simple liquids. We compare dispersions of "bare" acoustic and optic modes in NaCl with ab initio simulations of other ionic melts such as CuCl and LiBr, metallic liquid alloys such as Pb44Bi56 and Li4Tl, and the regular Lennard-Jones KrAr liquid simulated by classical molecular dynamics. Analytical expressions for the "bare" acoustic and optic branches of collective excitations help us to identify the impact of the high-frequency optic branch on the emergence of "fast sound" in binary melts. Our findings show that in ionic melts, the high-frequency speed of sound is much larger than in the simple Lennard-Jones liquids and metallic melts, leading to an observed strong viscoelastic increase in the apparent speed of sound-more than double its adiabatic value., (© 2024 Author(s). Published under an exclusive license by AIP Publishing.)

Transverse collective excitations in 50-50 and 80-20 Lennard-Jones binary liquid mixtures are studied for different mass ratio of components R at fixed numerical densities. Increasing the mass ratio results in a growing difference between frequencies of shear waves and transverse optic modes. We report an increase in the propagation gap width for shear waves with mass ratio of components and compare it to the gap width expression, known from the transverse dynamics of simple liquids. A four-variable dynamic model of transverse dynamics in binary liquids with an account of cross correlations between total-mass and mass-concentration transverse current fluctuations is solved analytically in the long-wavelength region. An equation for the propagation gap of shear waves for binary liquids is reported and analyzed., (© 2024 Author(s). Published under an exclusive license by AIP Publishing.)

Since decades, the concept of vibrational density of states in glasses has been mirrored in liquids by the instantaneous-normal-mode spectrum. In glasses instantaneous configurations are believed to be situated close to minima of the potential-energy hypersurface and all eigenvalues of the associated Hessian matrix are positive. In liquids this is no longer true, and modes corresponding to both positive and negative eigenvalues exist. The instantaneous-normal-mode spectrum has been numerically investigated in the past, and it has been demonstrated to bring important information on the liquid dynamics and transport properties. A systematic deeper theoretical understanding is now needed. Heterogeneous-elasticity theory has proven to be particularly successful in explaining many details of the low-frequency excitations in glasses, ranging from the thoroughly studied boson peak, to other anomalies related to the crossover between wave-like and random-matrix-like excitations. Here we present an extension of heterogeneous-elasticity theory to the liquid state, and show that the outcome of the theory agrees well to the results of extensive molecular-dynamics simulations of a model liquid at different temperatures. We find that the spectrum of eigenvalues [Formula: see text] has a sharp maximum close to (but not at) [Formula: see text], and decreases monotonically with [Formula: see text] on both its stable and unstable side. We show that the spectral shape strongly depends on temperature, being symmetric at high temperatures and becoming rather asymmetric at low temperatures, close to the dynamical critical temperature. Most importantly, we demonstrate that the theory naturally reproduces a surprising phenomenon, a zero-energy spectral singularity with a cusp-like character developing in the vibrational spectra upon cooling. This feature, known from a few previous numerical studies, has been generally overlooked in the past due to a misleading representation of the data. We provide a thorough analysis of this issue, based on both very accurate predictions of our theory, and computational studies of model liquid systems with extended size., (© 2023. The Author(s).)

One of the fundamental quantities in dynamics of the liquid state, the adiabatic speed of sound [Formula: see text], is extremely difficult to predict from computer simulations, especially in ab initio simulations. Here we derive an expression for the instantaneous correlator of fluctuations of longitudinal component of stress tensor, which contains [Formula: see text] along with others quantities easy accessible via classical and ab initio computer simulations. We show that the proposed methodology works well in the case of Lennard-Jones and soft-sphere simple fluids, Kr-Ar liquid mixture in connection with simulations with effective pair interactions as well as for liquid Sb, fluid Hg and molten NaCl from ab initio simulations., (© 2023. Springer Nature Limited.)

We report an ab initio simulation and theoretical study of collective dynamics in liquid Sb at 973 K. An application of the GCM (generalized collective modes) theoretical approach to analysis of simulation-derived time correlation functions resulted in two types of propagating eigenmodes. We found that the almost flat dispersion of the high-frequency branch of propagating modes can be explained by out-of-phase oscillations of nearest neighbors which form quasi-bound atomic pairs for at least 30 ps. We discuss the features of collective dynamics in non-simple metallic melts containing quasi-bound pairs., (© 2023 IOP Publishing Ltd.)

In a recent paper [Phys. Rev. E 104, 014103 (2021)10.1103/PhysRevE.104.014103] Baggioli and Zaccone formulate a theoretical description of the specific heat of liquids by using Debye's expression for the specific heat of solids and inserting a density of states which they claim to represent the instantaneous-normal-mode (INM) spectrum of a liquid. However, the quantum-mechanical procedure of Debye cannot be used for the relaxational excitations of a classical liquid. Furthermore, the authors' formula for the INM spectrum does not represent the known INM spectra of simple liquids, and the derivation of this formula from their model equation of motion is mathematically in error. These and a number of other inconsistencies render their work not very helpful for studying the specific heat of liquids.

We discuss a field-theoretical approach to liquids, solids, and glasses, published recently [Phys. Rev. E 105, 034108 (2022)10.1103/PhysRevE.105.034108], which aims to describe these materials in a common formalism. We argue that such a formalism contradicts the known hydrodynamic theory of classical liquids. In particular, the authors miss the important particle-number conservation law and the density fluctuations as a hydrodynamic slow variable. This results in an exotic mechanism of hydrodynamic sound instead of the standard hydrodynamic one due to combined particle-number and momentum conservation, a fact well documented in fluid-mechanics textbooks.