V. V. Stegailov

Stochastic theory of the classical molecular dynamics method

The work is devoted to fundamental aspects of the classical molecular dynamics method, which was developed half a century ago as a means of solving computational problems in statistical physics and has now become one of the most important numerical methods in the theory of condensed state. At the same time, the molecular dynamics method based on solving the equations of motion for a multiparticle system proved to be directly related to the basic concepts of classical statistical physics, in particular, to the problem of the occurrence of irreversibility. This paper analyzes the dynamic and stochastic properties of molecular dynamics systems connected with the local instability of trajectories and the errors of the numerical integration. The probabilistic nature of classical statistics is discussed. We propose a concept explaining the finite dynamic memory time and the emergence of irreversibility in real systems.

Standards for molecular dynamics modelling and simulation of relaxation

An attempt is made to formulate a set of requirements for simulation and modelling of relaxation in dense media. Each requirement is illustrated by examples of numerical simulation of particles with different types of interaction given by soft-sphere, Lennard–Jones, embedded atom method or Coulomb potential. The approaches developed are expected to be universal for some classes of relaxation processes in liquids, fluids, crystals and plasmas.

Interatomic potential for uranium in a wide range of pressures and temperatures

Using the force-matching method we develop an interatomic potential that allows us to study the structure and properties of α-U, γ-U and liquid uranium. The potential is fitted to the forces, energies and stresses obtained from ab initio calculations. The model gives a good comparison with the experimental and ab initio data for the lattice constants of α-U and γ-U, the elastic constants, the room-temperature isotherm, the normal density isochore, the bond-angle distribution functions and the vacancy formation energies. The calculated melting line of uranium at pressures up to 80 GPa and the temperature of the α-γ transition at 3 GPa agree well with the experimental phase diagram of uranium.

Melting and superheating of sI methane hydrate: molecular dynamics study.

Melting and decay of the superheated sI methane structure are studied using molecular dynamics simulation. The melting curve is calculated by the direct coexistence simulations in a wide range of pressures up to 5000 bar for the SPC/E, TIP4P/2005 and TIP4P/Ice water models and the united-atom model for methane. We locate the kinetic stability boundary of the superheated metastable sI structure that is found to be surprisingly high comparing with the predictions based on the classical nucleation theory.

Cavitation in liquid metals under negative pressures. Molecular dynamics modeling and simulation

An approach to study cavitation in stretched liquids via molecular dynamics (MD) simulation is presented. It is based on the stochastic properties of MD and allows one to study cavitation as a stochastic phenomenon. The approach is used to study equation of state and stability limits of the metastable liquid phase, cavitation kinetics and dynamics properties for different temperatures. Particular examples of metals under consideration include Pb, Li and Pb(83)Li(17). Quantitative and qualitative disagreements between the classic nucleation theory estimates and the MD results are found. The Kolmogorov-Johnson-Mehl-Avrami equation is used as an alternative way to estimate cavitation rate. The two methods show good mutual agreement. Decay at a constant stretching rate is also considered.

Nanomodification of gold surface by picosecond soft x-ray laser pulse

We show experimentally the possibility of nanostructuring (about 20 nm) of gold surface by picosecond soft x-ray single pulse with low fluence of ∼20 mJ/cm2. The nanometer-scale changes of the surface structure are due to the splash of molten gold under fluence gradient of the laser beam. In addition, the ablation process occurs at slightly higher fluence of ∼50 mJ/cm2. The atomistic model of ablation is developed which reveals that the low threshold fluence of this process is due to the build-up of the high electron pressure and the comparatively low electron-ion energy relaxation rate in gold. The calculated ablation depths as a function of the irradiation fluence are in good agreement with the experimental data measured for gold surface modification with ultra-short duration soft x-ray and visible lasers.

Atomistic simulation of laser ablation of gold: Effect of pressure relaxation

The process of ablation of a gold target by femto- and picosecond laser radiation pulses has been studied by numerical simulations using an atomistic model with allowance for the electron subsystem and the dependence of the ion-ion interaction potential on the electron temperature. Using this potential, it is possible to take into account the change in the physical properties of the ion subsystem as a result of heating of the electron subsystem. The results of simulations reveal a significant difference between the characteristics of metal ablation by laser pulses of various durations. For ablation with subpicosecond pulses, two mechanisms of metal fracture related to the evolution of electronic pressure in the system are established.

Molecular dynamics simulations of the relaxation processes in the condensed matter on GPUs

Abstract We report on simulation technique and benchmarks for molecular dynamics simulations of the relaxation processes in solids and liquids using the graphics processing units (GPUs). The implementation of a many-body potential such as the embedded atom method (EAM) on GPU is discussed. The benchmarks obtained by LAMMPS and HOOMD packages for simple Lennard-Jones liquids and metals using EAM potentials are presented for both Intel CPUs and Nvidia GPUs. As an example the crystallization rate of the supercooled Al melt is computed.

Surface melting of superheated crystals. Atomistic simulation study

Melting front velocity dependencies on temperature are calculated using the molecular-dynamics method for the EAM models of Al and Fe as well as for the Lennard-Jones system. Different surface orientations are considered. It is shown that the Broughton–Gilmer–Jackson theory of the collision-limited growth can describe the results obtained. The isochoric bulk solid melting and decay under ultrafast heating is simulated for mono- and polycrystalline models.

Molecular dynamics simulation of graphite melting

Questions on the behavior of the graphite melting curve have remained open during the last fifty years. The process of graphite melting in the pressure range of 2–14 GPa is investigated by the method of molecular dynamics using the model of reactive interatomic potential; the dynamics of melting-front propagation upon crystal superheating is considered, and the melting curve is plotted. The self-diffusion coefficient in the liquid phase is determined for the aforementioned pressure range, and the question of the existence of the liquid-liquid phase transition in carbon is considered.