Qing Hou

Molecular dynamics simulations with many-body potentials on multiple GPUs—The implementation, package and performance

Abstract Molecular dynamics (MD) is an important research tool extensively applied in materials science. Running MD on a graphics processing unit (GPU) is an attractive new approach for accelerating MD simulations. Currently, GPU implementations of MD usually run in a one-host-process-one-GPU (OHPOG) scheme. This scheme may pose a limitation on the system size that an implementation can handle due to the small device memory relative to the host memory. In this paper, we present a one-host-process-multiple-GPU (OHPMG) implementation of MD with embedded-atom-model or semi-empirical tight-binding many-body potentials. Because more device memory is available in an OHPMG process, the system size that can be handled is increased to a few million or more atoms. In comparison with the serial CPU implementation, in which Newton’s third law is applied to improve the computational efficiency, our OHPMG implementation has achieved a 28.9x–86.0x speedup in double precision, depending on the system size, the cut-off ranges and the number of GPUs. The implementation can also handle a group of small simulation boxes in one run by combining the small boxes into a large box. This approach greatly improves the GPU computing efficiency when a large number of MD simulations for small boxes are needed for statistical purposes.

Defect behavior induced by helium cluster growth in titanium crystals

The growth of helium cluster in titanium crystals is simulated in great detailed approach using molecular dynamics. We observe that, as the helium cluster grows, defects around the cluster are formed and the local pressure increases. However, at certain point in the growth process, the defects are found to rapidly escape as a whole from the helium cluster, accompanied by the relief of local high pressure and the recovery of Ti crystal structure around the helium cluster.

A modified W?W interatomic potential based on ab initio calculations

In this paper we have developed a Finnis?Sinclair-type interatomic potential for W?W interactions that is based on ab initio calculations. The modified potential is able to reproduce the correct formation energies of self-interstitial atom (SIA) defects in tungsten, offering a significant improvement over the Ackland?Thetford tungsten potential. Using the modified potential, the thermal expansion is calculated in a temperature range from 0 to 3500?K. The results are in reasonable agreement with the experimental data, thus overcoming the shortcomings of the negative thermal expansion using the Derlet?Nguyen?Manh?Dudarev tungsten potential. The W?W potential presented here is also applied to study in detail the diffusion of SIAs in tungsten. We reveal that the initial SIA initiates a sequence of tungsten atom displacements and replacements in the ?1?1?1? direction. An Arrhenius fit to the diffusion data at temperatures below 550?K indicates a migration energy of 0.022?eV, which is in reasonable agreement with the experimental data.

Cascade coalescence of noble gas bubbles in materials

Based on Monte Carlo simulations, we describe a mechanism, i.e., cascade-coalescence, to provide an explanation on the novel growth of gas bubbles in materials that has been observed in experiments and cannot be explained on the basis of the classical Smoluchowski theory and its variants. It is found that as the concentration and the average size of the bubbles reaches certain critical point, the cascade-coalescence is a dominant mechanism and leads to explosive growth of bubbles. The critical point for the cascade-coalescence happening can be evaluated by a scaling parameter determined by the average radius and concentration of the bubbles. This mechanism can be also used to explain the sudden release of gas atoms from thin films that can be measured in thermal desorption spectrometry in annealing experiments.

Theoretical calculations of energy transfer from noble gases to surfaces

Using the bipartition model of ion transport in a solid, we have calculated the energy reflection coefficients R(E) or equivalently the energy deposition coefficients F(E) for the ion‐target combinations of He‐Si, He‐Cu, He‐Ag, He‐Au, Ar‐Ag, Ar‐Au, and Xe‐Au. To test the validity of the theoretical method, we compared our results with corresponding experimental data and Monte Carlo data. The comparison shows that the results of the bipartition model reach a reasonable agreement with experimental data and Monte Carlo data.

Diffusion behavior of helium in titanium and the effect of grain boundaries revealed by molecular dynamics simulation

The microstructures of titanium (Ti), an attractive tritium (T) storage material, will affect the evolution process of the retained helium (He). Understanding the diffusion behavior of He at the atomic scale is crucial for the mechanism of material degradation. The novel diffusion behavior of He has been reported by molecular dynamics (MD) simulation for the bulk hcp-Ti system and the system with grain boundary (GB). It is observed that the diffusion of He in the bulk hcp-Ti is significantly anisotropic (the diffusion coefficient of the [0001] direction is higher than that of the basal plane), as represented by the different migration energies. Different from convention, the GB accelerates the diffusion of He in one direction but not in the other. It is observed that a twin boundary (TB) can serve as an effective trapped region for He. The TB accelerates diffusion of He in the direction perpendicular to the twinning direction (TD), while it decelerates the diffusion in the TD. This finding is attributable to the change of diffusion path caused by the distortion of the local favorable site for He and the change of its number in the TB region.

A molecular dynamics study of melting and dissociation of tungsten nanoparticles

Molecular dynamics simulations were conducted to study the melting and dissociation of free tungsten nanoparticles. For the various interatomic potentials applied, the melting points of the tungsten nanoparticles increased with increasing nanoparticle diameter. Combining these results with the melting point of bulk tungsten in the experiment, the melting point of nanoparticles with diameters ranging from 4 to 12 nm could be determined. As the temperature increases, free nanoparticles are subject to dissociation phenomena. The dissociation rate was observed to follow Arrhenius behavior, and the Meyer–Neldel rule was obeyed. These results are useful in understanding the behavior of tungsten dust generated in nuclear fusion devices as well as for the preparation, formation, and application of tungsten powders.

Universal expressions for range parameters of ions in solids

The detailed parametric analysis of the Boltzmann equation of ion transport shows that there is an approximately one‐to‐one correspondence between ion transport quantities and a characteristic parameter named the scaled transport cross section and defined earlier [Z. Luo, Nucl. Instrum. Methods B 48, 444 (1990)]. To attest to a significant conclusion, we have completed systematic calculations over numerous‐ranges for ions implanted in solids. The systematic calculations include 3410 ion‐target‐energy combinations. The calculations show that range parameters present a good single‐value dependence on the scaled transport cross section at low incident energies. Based on this result, universal expressions for range parameters have been obtained. To test the validity of the expressions, the range distributions of ions in solids have been calculated by universal expressions. The comparisons between the results of the universal expressions and experimental or Monte Carlo data show that the universal expressions ...

REFLECTION OF LIGHT IONS FROM HEAVY ELEMENT TARGETS

An improved bipartition theory for light‐ion transport is presented. The improved theory allows the use of accurate but more complex nuclear scattering cross sections and stopping powers in the Boltzmann transport equation, instead of the power function approximation of the nuclear scattering cross sections and the stopping powers used in earlier bipartion theory [Z.‐M. Luo and S.‐M. Wang, Phys. Rev. B 36, 1885 (1987)]. Furthermore, the theory is extended to treat the transport for obliquely incident ions. By the improved bipartition theory, the particle reflection coefficients and energy reflection coefficients, as well as energy distribution of reflected ions for cases of H, D, and He ions incident on C, Ni, Fe, Cu, W, and Au, have been calculated. The comparison of the present calculation results with available experimental data and the Monte Carlo data shows that the improved bipartition model for light ions is useful.

Energy deposition calculations in dose measurements of 0.2–3.0 MeV electrons ☆

Abstract In this paper emphasis is laid upon the energy deposition calculations of 0.3–0.6 MeV low-energy electrons in multilayer media. In addition the energy depositions of 0.2–3.0 MeV electrons in some materials have been calculated by the bipartition model of electron transport. The calculation results are in agreement with some experimental data. The energy deposition values of 0.3–0.6 MeV electrons have been fitted into an approximate calculation formula which can be used for dose measurements and calculations of low-energy electron beam radiation curing and film material radiation processing.