arXiv subjects

P. R. Levashov

Publications and source records attributed to P. R. Levashov.

At least 19 recordsLinked to original sources

arXiv2026

Pressure-dependent melting and crystallization of B2-NiAl from neural-network molecular dynamics

We investigate pressure-dependent melting of ordered B2-NiAl using neural-network molecular dynamics with a Deep Potential interatomic model. Melting temperatures are determined from two-phase solid-liquid coexistence simulations over a broad pressure range, yielding the melting curve $T_m(P)$. Relative to available experimental and previous molecular-dynamics results, the present calculations predict a stronger increase of the melting temperature with pressure at elevated compression. To assess the thermodynamic consistency of the calculated melting line, we evaluate the enthalpy and volume changes upon melting and compare the Clapeyron slope with the derivative of the fitted $T_m(P)$ curve. The two estimates are in good agreement over most of the investigated pressure range, supporting the internal consistency of the coexistence results. To probe the character of melting, we perform a layer-resolved composition analysis of the coexistence configurations and find that the coexisting liquid remains essentially equiatomic at all studied pressures, with deviations of the aluminum fraction from the stoichiometric value not exceeding $5\times10^{-3}$. This provides direct atomistic evidence that melting of B2-NiAl remains congruent within the present model. Together, these results establish a thermodynamically consistent pressure-dependent melting description of B2-NiAl and clarify the character of its melting under compression.

cond-mat.mtrl-sci↗

arXiv2026

A wide-range temperature-dependent deep potential for sodium with near-experimental accuracy from melting to the critical point

We construct temperature-dependent deep potentials for sodium using finite-temperature DFT data obtained with the PBE, AM05, and $r^2$SCAN exchange-correlation functionals and investigate the thermophysical properties of sodium from room temperature to the critical region. The predicted critical parameters show a dependence on the exchange-correlation functional. The model based on the $r^2$SCAN functional gives a critical temperature $T_c=2.508(8)$ kK, density $ρ_c=0.203(4)$ g/cm$^{3}$, and pressure $P_c=0.249(6)$ kbar, in close agreement with the recommended values. This model is then used to reconstruct the normal-pressure and critical isobars and to calculate the enthalpy, heat capacities, thermal expansion coefficient, bulk moduli, Grüneisen parameter, and speed of sound. The calculated normal-pressure bcc density differs by only 0.06% from the experimental value. Direct solid-liquid coexistence simulations give a melting temperature of $346(2)$ K, 25 K below the recommended value, consistent with the sensitivity expected from meV/atom free-energy errors. Liquid-vapor coexistence simulations reproduce the binodal and yield a surface tension that approaches zero near the critical point. The calculated self-diffusion coefficient and shear viscosity extend the available transport-property data into the expanded-liquid and near-critical regions, where direct experimental information is sparse. We also demonstrate the thermodynamic consistency of the results by comparing the speed of sound obtained from direct acoustic simulations with that calculated from the equation of state.

physics.comp-ph↗

arXiv2025

MDcraft -- a modern molecular dynamics simulation package with machine learning potentials support

Molecular dynamics is widely used to study various phenomena, such as diffusion, shock wave propagation, and plasma dynamics. A wide range of software packages supports the expanding scope of molecular dynamics applications. However, the quality of simulations depends on force field approximations, ranging from simple models to direct quantum solutions. Recently, machine learning approaches for constructing accurate interatomic potentials have received significant attention. In MDcraft, we integrate these advances into a scalable, physically accurate framework. MDcraft is a comprehensive, modern molecular dynamics platform. It offers a high-level Python API with a user-friendly, script-based interface. The core simulation algorithms are implemented in C++ to ensure robustness and computational efficiency. MDcraft is built for high-performance computing on modern clusters and supports dynamic domain decomposition and load balancing via the Message Passing Interface (MPI) for scalable parallelization. Additionally, MDcraft leverages multithreading within nodes through standard C++ parallelism, enabling efficient use of heterogeneous architectures. We demonstrate the code's capabilities through several examples, including the shock response in aluminum, the shock Hugoniot in argon, and the cold curve of copper.

physics.comp-ph↗

arXiv2025

One--Component Plasma Equation of State Revisited via Angular--Averaged Ewald Potential

We present analytic fits of classical one--component plasma (OCP) internal energy over a wide range of coupling parameter $0.01\leΓ\le 170$ using Monte--Carlo data in the thermodynamic limit. We extend the dataset obtained in [Demyanov and Levashov, Phys. Rev. E 106, 015204 (2022)] using the angular--averaged Ewald potential with additional points at strong coupling ($Γ=120,\ 150,\ 170$). We then fit two frequently used functional forms for the OCP equation of state: (i) a five-parameter equation by Caillol [J. Chem. Phys. 111, 6538--6547 (1999)] and (ii) the equation by Potekhin and Chabrier [Phys. Rev. E 62, 8554 (2000)] that enforces the Debye--Hückel limit. The presented fits reproduce our MC data within statistical uncertainties, recovering the correct weak-coupling behavior. Coefficients, recommended validity ranges, and comparisons to prior analytical and simulation results are provided.

physics.plasm-ph↗

arXiv2025

Molecular dynamics of nondegenerate hydrogen plasma using improved Kelbg pseudopotential with electron finite-size correction

This paper is devoted to semiclassical molecular dynamics simulation of nondegenerate hydrogen plasma using an improved Kelbg pseudopotential. The main novelty of our method is accounting for the finite size of electrons. This modification resolves the nonphysical cluster formation at temperatures below 50 kK, which was first reported by A.V. Filinov [Phys. Rev. E 70, 046411 (2004)]. However, the energy still appears to be underestimated at low temperatures, as indicated by comparisons with the recent path integral Monte Carlo calculations [Phys. Plasmas 31, 110501 (2024)]. Using the presented method, we analyze the dependence of radial distribution functions, composition, ionization degree, energy, and pressure on the plasma coupling parameter, while maintaining a fixed degeneracy parameter. Additionally, we demonstrate the impact of incorporating long-range interactions on the energy $N$-dependence by utilizing the angular-averaged Ewald potential. Finally, we compute the thermodynamic limits for energy and pressure.

physics.plasm-ph↗

arXiv2024

Phase space path integral representation of the dynamic structure factor. Monte Carlo simulation of strongly correlated soft-sphere fermions

The dynamic structure factor (DSF) is a mathematical function that contains information about inter-particle correlations and their time evolution. Mostly the classical molecular dynamics is used to calculate the DSF of the classical systems. On the contrary this article deals with quantum systems and the quantum dynamic structure factor. The Wigner formulation of quantum mechanics was used to derive the path integral representation of the DSF, which is based on the Wiener-Khinchin theorem showing relation of the the power spectrum of a random paths to their correlation function. The $3 {\rm D} $ quantum system of strongly correlated soft-sphere fermions was considered as an interesting physical example. The developed Wigner path integral Monte Carlo (WPIMC) approach has been developed to calculate the spin--resolved DSFs, the radial distribution functions (RDFs) and other thermodynamic functions in a wide range of density and temperatures. The physical meaning of the peaks arising on the RDFs and DSF have been analyzed and explained by the manifestation of the interference effects of the exchange and interparticle interactions and, as well as, the wave interference between multiple-scattering. This phenomenon in the system of the soft-sphere scatterers may to be the precursor effect of the Anderson localization, which finds its origin in the wave interference between multiple-scattering paths.

cond-mat.dis-nn↗

arXiv2023

Stability of a Nondegenerate Two--Component Weakly Coupled Plasma

The paper discusses the problem of stability of a two-component plasma and proposes a consistent consideration of quantum and long-range effects to calculate the thermodynamic properties of such a plasma. We restrict ourselves by the case of a non-degenerate plasma to avoid the fermionic sign problem and consider the weakly coupled regime. Long-range interaction effects are taken into account using the angular-averaged Ewald potential (AAEP). To calculate thermodynamic properties, we apply both classical Monte Carlo (CMC) and path integral Monte Carlo (PIMC) methods. A special method is developed to correctly calculate potential energy in PIMC simulations with long-range interaction effects. Our theoretical estimations show that the probability of a bound state formation is very low at a coupling parameter $Γ~\le~0.01$, so both classical and quantum simulations give the same energy at $Γ~\leq~0.01$, and the thermodynamic limit coincides with the Debye--Hückel theory. At higher $Γ$, the $N$-convergence is lost due to the formation of bound states.

physics.plasm-ph↗

arXiv2023

Pressure of Coulomb systems with volume-dependent long-range potentials

In this work, we consider the pressure of Coulomb systems, in which particles interact via a volume-dependent potential (in particular, the Ewald potential). We confirm that the expression for virial pressure should be corrected in this case. We show that the corrected virial pressure coincides with the formula obtained by differentiation of free energy if the potential energy is a homogeneous function of particle coordinates and a cell length. As a consequence, we find out that the expression for pressure in the recent paper by J. Liang \textit{et al.} [\href{https://doi.org/10.1063/5.0107140}{J. Chem. Phys. \textbf{157}, 144102 (2022)}] is incorrect.

physics.comp-ph↗

arXiv2023

Exchange--correlation bound states of the triplet soft--sphere fermions by the path integral Monte Carlo simulations

Path integral Monte Carlo simulations in the Wigner approach to quantum mechanics has been applied to calculate momentum and spin--resolved radial distribution functions of the strongly correlated soft--sphere quantum fermions. The obtained spin--resolved radial distribution functions demonstrate arising triplet clusters of fermions, that is the consequence of the interference of exchange and interparticle interactions. The semiclassical analysis in the framework of the Bohr--Sommerfeld quantization condition applied to the potential of the mean force corresponding to the same--spin radial distribution functions allows to detect exchange--correlation bound states in triplet clusters and to estimate corresponding averaged energy levels. The obtained momentum distribution functions demonstrate the narrow sharp separated peaks corresponding to bound states and disturbing the Maxwellian distribution.

physics.comp-ph↗

arXiv2023

A wide-range semiclassical self-consistent average atom model

Discovery of material properties at extremes, which are essential for high energy density physics development, requires the most advanced experimental facilities, theories, and computations. Nowadays it is possible to model properties of matter in such conditions using the state-of-the-art density functional theory (DFT) or path-integral Monte--Carlo (PIMC) approaches with remarkable precision. However, fundamental and computational limitations of these methods impede their practical usage while wide-range thermodynamic and transport models of plasma are required. As a consequence, an average atom (AA) framework is still relevant today and has been attracting more and more attention lately. The self-consistent field and electron density in an atomic cell is usually obtained using the Thomas--Fermi (TF), Hartree--Fock (HF), Kohn--Sham (KS) approaches, or their extensions. In this study we present the AA model, where semiclassical wave functions are used for bound states, while free electrons are approximated by the TF model with a thermodynamically consistent energy boundary. The model is compared in various regions of temperatures and pressures with the reference data: Saha model for rarefied plasma, DFT for warm dense matter, and experimental shock Hugoniot data. It is demonstrated that a single AA model may provide a reasonable agreement with the established techniques at low computational cost and with stable convergence of the self-consistent field.

physics.plasm-ph↗

arXiv2022

One--Component Plasma of a Million Particles via angular--averaged Ewald potential: A Monte Carlo study

In this work, we derive a correct expression for the one--component plasma (OCP) energy via the angular--averaged Ewald potential (AAEP). Unlike E.~Yakub and C.~Ronchi (J. Low Temp. Phys. 139, 633 (2005)), who had tried to obtain the same energy expression from a two--component plasma model, we used the original Ewald potential for an OCP. A constant in the AAEP was determined using the cluster expansion in the limit of weak coupling. The potential has a simple form suitable for effective numerical simulations. To demonstrate the advantages of the AAEP, we performed a number of Monte--Carlo simulations for an OCP with up to a million particles in a wide range of the coupling parameter. Our computations turned out at least two orders of magnitude more effective than those with a traditional Ewald potential. A unified approach is offered for the determination of the thermodynamic limit in the whole investigated range. Our results are in good agreement with both theoretical data for a weakly coupled OCP and previous numerical simulations. We hope that the AAEP will be useful in path integral Monte Carlo simulations of the uniform electron gas.

physics.plasm-ph↗

arXiv2022

Systematic derivation of angular--averaged Ewald potential

In this work we provide a step by step derivation of an angular--averaged Ewald potential suitable for numerical simulations of disordered Coulomb systems. The potential was first introduced by E.\,Yakub and C.\,Ronchi without a clear derivation. Two methods are used to find the coefficients of the series expansion of the potential: based on the Euler--Maclaurin and Poisson summation formulas. The expressions for each coefficient is represented as a finite series containing derivatives of Jacobi theta functions. We also demonstrate the formal equivalence of the Poisson and Euler--Maclaurin summation formulas in the three-dimensional case. The effectiveness of the angular--averaged Ewald potential is shown by the example of calculating the Madelung constant for a number of crystal lattices.

physics.plasm-ph↗

arXiv2022

Derivation of the Kelbg potential/functional

The density matrix for a system of particles interacting via the Coulomb potential is obtained in the high--temperature limit following almost entirely the original work by Kelbg. For this purpose the Blöch equation is solved in the first order of perturbation theory. We tried to explain all the transformations in the derivation in order to simplify the understanding of this non-trivial theory. The solution of Kelbg is widely used in path integral simulations of Coulomb systems.

physics.plasm-ph↗

arXiv2022

Momentum distribution functions and pair correlation functions of unpolarized uniform electron gas in warm dense matter regime

In this paper we continued our research of the uniform electron gas, using the single--momentum path integral Monte Carlo method, and studied the momentum distribution functions and the pair correlation functions in the warm dense matter regime. We discovered that the single--particle momentum distribution function deviates from the Fermi distribution and forms so-called "quantum tails" at high momenta, if non-ideality is strong enough in both degenerate and non-degenerate cases. This effect is always followed by the appearance of the short--range order on the pair distribution functions and can be explained via the tunneling through the effective potential wells surrounding the electrons. Also we calculated the average kinetic and potential energies in the wide range of states, expanding our previous results significantly.

cond-mat.stat-mech↗

arXiv2021

Ab initio inspection of thermophysical experiments for zirconium near melting

We present quantum molecular dynamics calculations of thermophysical properties of solid and liquid zirconium in the vicinity of melting. An overview of available experimental data is also presented. We focus on the analysis of thermal expansion, molar enthalpy, resistivity and normal spectral emissivity of solid and liquid Zr. Possible reasons of discrepancies between the first-principle simulations and experiments are discussed. Our calculations reveal a significant volume change on melting in agreement with electrostatic levitation experiments. Meanwhile, we confirm a low value of enthalpy of fusion obtained in some pulse-heating experiments. Electrical resistivity of solid and liquid Zr is systematically underestimated in our simulations, however the slope of resistivity temperature dependencies agrees with experiment. Our calculations predict almost constant normal spectral emissivity in liquid Zr.

cond-mat.mtrl-sci↗

arXiv2020

Effect of the spin-orbit interaction on thermodynamic properties of liquid uranium

We present the first quantum molecular dynamics calculation of zero-pressure isobar of solid and liquid uranium that account for spin-orbit coupling. We demonstrate that inclusion of spin-orbit interaction leads to higher degree of the thermal expansion of uranium, especially in the liquid phase. Full accounting of relativistic effects for valence electrons, particularly spin-orbital splitting of the 5f band, is substantial for the reproduction of the experimental density of molten uranium at the melting temperature. Influence of the spin-orbit interaction on the thermodynamic properties at high temperatures and pressures is also analyzed.

cond-mat.mtrl-sci↗

arXiv2017

Structural, thermodynamic, and transport properties of CH$_2$ plasma in the two-temperature regime

This paper covers calculation of radial distribution functions, specific energy and static electrical conductivity of CH$_2$ plasma in the two-temperature regime. The calculation is based on the quantum molecular dynamics, density functional theory and the Kubo-Greenwood formula. The properties are computed at 5 kK $\le$ $T_i$ $\le$ $T_e$ $\le$ 40 kK and ρ= 0.954 g/cm$^3$ and depend severely on the presence of chemical bonds in the system. Chemical compounds exist at the lowest temperature $T_i$ = $T_e$ = 5 kK considered; they are destroyed rapidly at the growth of $T_i$ and slower at the increase of $T_e$. A significant number of bonds are present in the system at 5 kK $\le$ $T_i$ $\le$ $T_e$ $\le$ 10 kK. The destruction of bonds correlates with the growth of specific energy and static electrical conductivity under these conditions.

physics.plasm-ph↗

arXiv2016

Ab initio calculation of thermodynamic, transport, and optical properties of CH$_2$ plastics

This work covers an ab initio calculation of thermodynamic, transport, and optical properties of plastics of the effective composition CH$_2$ at density 0.954 g/cm$^3$ in the temperature range from 5 kK up to 100 kK. The calculation is based on the quantum molecular dynamics, density functional theory and the Kubo-Greenwood formula. The temperature dependence of the static electrical conductivity $σ(T)$ has a step-like shape: $σ(T)$ grows rapidly for 5 kK <= $T$ <= 10 kK and is almost constant for 20 kK <= $T$ <= 60 kK. The additional analysis based on the investigation of the electron density of states (DOS) is performed. The rapid growth of $σ(T)$ at 5 kK<= $T$ <= 10 kK is connected with the increase of DOS at the electron energy equal to the chemical potential $ε= μ$. The frequency dependence of the dynamic electrical conductivity $σ_1(ω)$ at 5 kK has the distinct non-Drude shape with the peak at $ω\approx 10$ eV. This behavior of $σ_1(ω)$ was explained by the dip at the electron DOS.

cond-mat.mtrl-sci↗