We present the results of molecular dynamics simulations of the crystal nucleation rate in a supercooled Lennard-Jones liquid. The nucleation rate as a function of the pressure has been calculated by the method of determining the expectation time for liquid crystallization at temperatures higher than that of the triple point (T(*) = 0.865), close to the temperature of the terminal critical point of the metastable extension of the melting curve (T(*) = 0.55) and below this temperature (T(*) = 0.4). In computer experiments the nucleation rate varied from 10(32) to 10(35) s(-1) m(-3). The dimensions of critical nuclei and the pressure inside them, the surface free energy at a critical crystal nucleus-liquid interface, the height of the nucleation barrier, and the Zeldovich factor have been determined from the results of molecular dynamics simulations and their comparison with classical homogeneous nucleation theory. It is shown that the surface free energy at a curved crystal-liquid interface, as distinct from a flat interface, has also been determined at temperatures lower than the temperature of the terminal critical point of the melting curve and is a monotonically increasing function of the temperature.

Numerical simulations were carried out to model the propagation of an airwave from the fireball that passed over Chelyabinsk (Russia) on 15 February 2013. The airburst of the Chelyabinsk meteoroid occurred due to its catastrophic fragmentation in the atmosphere. Simulations of the space-time distribution of energy deposition during the airburst were done using a novel fragmentation model based on dimensionality considerations and analogy to the fission chain reaction in fissile materials. To get an estimate of the airburst energy, observed values of the airwave arrival times to different populated localities were retrieved from video records available on the Internet. The calculated arrival times agree well with the observed values for all the localities. Energy deposition in the atmosphere obtained from observations of the airwave arrival times was found to be 460 ± 60 kt in trinitrotoluene (TNT) equivalent. We also obtained an independent estimate for the deposited energy,450 þ200 À160 kt TNT from detecting the air increment velocity due to the wave passage in Chelyabinsk. Assuming that the energy of about 90 kt TNT was irradiated in the form of visible light and infrared radiation, as registered with optical sensors [Yeomans and Chodas, 2013], one can value the total energy release to be about 550 kt TNT which is in agreement with previous estimates from infrasound registration and from optical sensors data. The overpressure amplitude and its positive phase duration in the airwave that reached the city of Chelyabinsk were calculated to be about 2 kPa and 10 s accordingly.

Analysis of phase diagram of beryllium at high pressures and temperatures obtained as a result of ab initio calculations and large scale classical molecular dynamics simulations of beryllium shock loading have shown that the so called cold melting takes place when shock wave propagates through polycrystalline samples. Comparison of ab initio calculation results on sound speed along the Hugoniot with experimental data obtained on Z-machine also evidences for possible manifestation of the cold melting. The last may explain the discrepancy between atomistic simulations and experimental data on the onset of the melting on the Hugoniot.

No abstract

No abstract

No abstract

Abstract. Theoretical and experimental investigation into properties of condensed matter is one of the mainstreams in RFNC-VNIITF scientific activity. The method of molecular dynamics (MD) is an innovative method of theoretical materials science. Modern supercomputers allow the direct simulation of collective effects in multibillion atom sample, making it possible to model physical processes on the atomistic level, including material response to dynamic load, radiation damage, influence of defects and alloying additions upon material mechanical properties, or aging of actinides. During past ten years, the computer code MOLOCH has been developed at RFNC-VNIITF. It is a parallel code suitable for massive parallel computing. Modern programming techniques were used to make the code almost 100% efficient. Practically all instruments required for modelling were implemented in the code: a potential builder for different materials, simulation of physical processes in arbitrary 3D geometry, and calculated data processing. A set of tests was developed to analyse algorithms efficiency. It can be used to compare codes with different MD implementation between each other.

No abstract