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T. Bryk

Publications and source records attributed to T. Bryk.

18 recordsLinked to original sources
arXiv2025

Collective excitations in Hydrogen across the pressure-induced transition from molecular to atomic fluid

Dispersion of collective excitations in fluid Hydrogen along the isothermal line T=2500~K, including the region of molecular-to-atomis fluid transition, is studied by ab initio molecular dynamics (AIMD) simulations. The obtained density dependence of the adiabatic and high-frequency speed of sound contains a plateau in the region of the molecular-to-atomic fluid transition. We show, that the five-variable thermo-viscoelastic model of generalized hydrodynamics for pure molecular H$_2$ and pure atomic (H) fluids is able to recover perfectly the AIMD-derived time correlation functions and sound eigenvalues nicely agree with the numerically estimated sound dispersion. In the region of the molecular-to-atomic fluid transition a dynamic model of chemical reacting mixture should be applied. We discuss the calculations of time correlation functions from molecular/atomic units in the reacting mixture from AIMD trajectories.

physics.chem-ph↗
arXiv2025

Elastic properties of fluid mercury across the metal-nonmetal transition: Ab initio simulation study

We report an ab initio molecular dynamics study of fluid mercury at temperature 1750 K in the range of densities 7-13.5 g/cm$^3$. Along this isothermal line we performed an analysis of total charge fluctuations, which make evidence of neutral atom-like screening in fluid Hg for densities less than 9.25 g/cm$^3$, which practically coincides with the emergence of the gap in electronic density of states. High-frequency shear modulus, high-frequency and adiabatic speeds of sound, shear viscosity, Maxwell relaxation time and dispersion of collective excitations are analyzed as a function of density along the isothermal line.

cond-mat.dis-nn↗
arXiv2025

Bimodality of local structural ordering in extremely confined hard disks

By combining computer simulations and a unit cell model approach, we study the apparent bimodality of local structural ordering in a system of confined hard disks. It is shown that a two-dimensional (2D) array of hard disks confined laterally within a quasi-1D hard wall channel of the width commensurate with the bulk 2D triangular lattice at disk close packing, possesses a bimodal probability distribution for the distance between disks' left and right nearest neighbors. The observed feature aligns with the concept of locally favored structures intensively exploited in the studies of anomalous thermodynamic and kinetic behavior of hydrogen-bonding fluids, except that the reported case is driven by entropic bonding only. The bimodality is observed in a range of densities associated with the vicinity of freezing transition in bulk 2D hard disks, indicating a crossover from the "gas-like" to "liquid-like" state in confined quasi-1D hard disks. Such a phenomenon was not reported for bulk 2D hard disks and is physically unexpected for confined q1D hard disks.

cond-mat.soft↗
arXiv2023

Canonical partition function and distance dependent correlation functions of a quasi-one-dimensional system of hard disks

The canonical NLT partition function of a quasi-one dimensional (q1D) one-file system of equal hard disks [J. Chem Phys. 153, 144111 (2020)] provides an analytical description of the thermodynamics and ordering in this system (a pore) as a function of linear density Nd/L where d is the disk diameter. We derive the analytical formulae for the distance dependence of the translational pair distribution function and the distribution function of distances between next neighbor disks, and then demonstrate their use by calculating the translational order in the pore. In all cases, the order is found to be of a short range and to exponentially decay with the disks' separation. The correlation length presented for different pore widths and densities shows a non-monotonic dependence with a maximum at Nd/L=1 and tends to the 1D value for a vanishing pore width. The results indicate a special role of this density when the pore length L is equal exactly to N disk diameters. A comparison between the theoretical results for an infinite system and the results of a molecular dynamics simulation for a finite system with periodic boundary conditions is presented and discussed.

cond-mat.dis-nn↗
arXiv2021

Absence of a boson peak in anharmonic phonon models with Akhiezer-type damping

In a recent article M. Baggioli and A. Zaccone (Phys. Rev. Lett. {\bf 112}, 145501 (2019)) claimed that an anharmonic damping, leading to a sound attenuation proportional to $\omega^2$ (Akhiezer-type damping) would imply a boson peak, i.e.\ a maximum in the vibrational density of states, divided by the frequency squared (reduced density of states). This would apply both to glasses and crystals.Here we show that this is not the case. In a mathematically correct treatment of the model the reduced density of states monotonously decreases, i.e.\ there is no boson peak. We further show that the formula for the would-be boson peak, presented by the authors, corresponds to a very short one-dimensional damped oscillator system. The peaks they show correspond to resonances, which vanish in the thermodynamic limit.

cond-mat.dis-nn↗
arXiv2020

Structural and dynamic features of liquid Si under high pressure above the melting line minimum

We report an {\it ab initio} simulation study of changes in structural and dynamic properties of liquid Si at 7 pressures ranging from 10.2 GPa to 24.3 GPa along the isothermal line 1150~K, which is above the minimum of the melting line. The increase of pressure from 10.2 GPa to 16 GPa causes strong reduction in the tetrahedral ordering of the most close neighbors. The diffusion coefficient shows a linear decay vs drop in atomic volume, that agrees with theoretical prediction for simple liquid metals, thus not showing any feature at the pressures corresponding to the different crystal phase boundaries. The Fourier-spectra of velocity autocorrelation function shows two-peak structure at pressures 20 GPa and higher. These characteristic frequencies correspond well to the peak frequencies of the transverse current spectral function in the second pseudo-Brillouin zone. Two almost flat branches of short-wavelength transverse modes were observed for all the studied pressures. We discuss the pressure evolution of characteristic frequencies in the longitudinal and transverse branches of collective modes.

cond-mat.dis-nn↗
arXiv2020

Velocity autocorrelations across the molecular-atomic fluid transformation in hydrogen under pressure

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.

cond-mat.soft↗
arXiv2019

Detection of collective optic excitations in molten NaI

High-resolution inelastic x-ray scattering measurements were carried out on molten NaI near the melting point at 680$^\circ$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, $Q\sim5$ nm$^{-1}$. The measured spectra are in good agreement, in both frequency and linewidth, with {\it 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.

cond-mat.mtrl-sci↗
arXiv2019

A simple closure procedure for the study of velocity autocorrelation functions in fluids as a "bridge" between different theoretical approaches

Velocity autocorrelation functions (VAF) of the fluids are studied on short- and long-time scales within a unified approach. This approach is based on an effective summation of the infinite continued fraction at a reasonable assumption about convergence of relaxation times of the high order memory functions, which have purely kinetic origin. The VAFs obtained within our method are compared with computer simulation data for the liquid Ne at different densities and the results, which follow from the Markovian approximation for the highest order kinetic kernels. It is shown that in all the thermodynamic points and at the chosen level of the hierarchy, our results agree much better with the MD data than those of the Markovian approximation. The density dependence of the transition time, needed for the fluid to attain the hydrodynamic stage of evolution, is evaluated. The common and distinctive features of our method are discussed in their relations to the generalized collective mode (GCM) theory, the mode coupling theory (MCT), and some other theoretical approaches.

cond-mat.stat-mech↗
arXiv2018

A simple ansatz for the study of velocity autocorrelation functions in fluids at different timescales

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.

cond-mat.stat-mech↗
arXiv2016

Ab initio molecular dynamics study of collective excitations in liquid H$_2$O and D$_2$O: Effect of dispersion corrections

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$^\circ$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$^{-1}$).

cond-mat.soft↗
arXiv2016

Profiles of electrostatic potential across the water-vapor, ice-vapor and ice-water interfaces

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.

cond-mat.soft↗
arXiv2015

Heat capacity of liquids: A hydrodynamic approach

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, $C_{v}(k)$ and $C_{p}(k)$, are shown to be in the long-wavelength limit in good agreement with the macroscopic experimental values of $C_{v}$ and $C_{p}$ for the studied thermodynamic points of supercritical Ar.

cond-mat.soft↗