Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Physical Review B

Interpretation of X-ray Powder Diffraction Patterns . By H. Lipson and H. Steeple. Pp. viii + 335 + 3 plates. (Mac-millan: London; St Martins Press: New York, May 1970.) £4.

X-Ray Diffraction

The article tells about the diversity of green spaces in London, the citizens’ attitude toward these spaces, and what Russian cities can learn from this experience.

Nature in London

Knowledge and foundational understanding of phenomena associated with the behavior of materials at the nanoscale is one of the key scientific challenges toward a sustainable energy future. Size reduction from bulk to the nanoscale leads to a variety of exciting and anomalous phenomena due to enhanced surface-to-volume ratio, reduced transport length, and tunable nanointerfaces. Nanostructured metal hydrides are an important class of materials with significant potential for energy storage applications. Hydrogen storage in nanoscale metal hydrides has been recognized as a potentially transformative technology, and the field is now growing steadily due to the ability to tune the material properties more independently and drastically compared to those of their bulk counterparts. The numerous advantages of nanostructured metal hydrides compared to bulk include improved reversibility, altered heats of hydrogen absorption/desorption, nanointerfacial reaction pathways with faster rates, and new surface states cap...

Nanostructured Metal Hydrides for Hydrogen Storage.

A molecular dynamics simulation study has been performed to investigate the solid microstructures of Ag at room temperature resulted from rapid solidification with six cooling rates by using Quantum Sutton–Chen (QSC) many body potential. Several structural analysis methods have been employed to characterize and analyze the atomic configurations in the system. The results reveal that as the cooling rate γ ⩾ 1.0 × 10 13  K/s, an amorphous structure can be obtained; as the cooling rates in the range of γ = 5.0 × 10 12  ∼ 1.38 × 10 11  K/s, the coexistence structures of the meta-stable hcp with stable fcc configurations can be formed with phase separating or layering structures. Moreover, by using the 3D graphics technology, the arrangement of atoms in the system reveals that the phase separating occurs before the layering structures can be formed following the cooling rate decreasing. And an interesting layering structures have been found, in which the central atoms of fcc and hcp basic clusters are arranged by alternate close-packed-layers with a new repeating sequence ABCB.

Molecular dynamics simulation for cooling rate dependence of solidification microstructures of silver

A molecular dynamics simulation study has been performed for a large-sized system consisting of 10 6 liquid metal Al atoms to investigate the formation and magic number characteristics of various clusters formed during solidification processes. The cluster-type index method (CTIM) is adopted to describe various types of cluster by basic clusters. It is demonstrated that the icosahedral cluster (12 0 12 0) is the most important basic cluster, and that it plays a critical role in the microstructure transition. A new statistical method has been proposed to classify the clusters as some group levels according to the numbers of basic clusters contained in each cluster. The magic numbers can be determined by the respective peak value positions of different group levels of clusters, and the magic number sequence in the system is 13, 19, 25(27), 31(33), 38(40), 42(45), 48(51), 55(59), 61(65), 67 ,... the numbers in the brackets are the second magic number of the corresponding group levels of clusters. This magic number sequence is in good agreement with the experimental results obtained by Schriver and Harris et al, and the experimental results can be reasonably well explained. (Some figures in this article are in colour only in the electronic version)

http://iopscience.iop.org/article/10.1088/0953-8984/19/19/196103/pdf

Formation and magic number characteristics of clusters formed during solidification processes

Structural stabilities, elastic properties and electronic structures have been determined from first-principles calculation by using Castep and Dmol program based on the density functional theory for Mg 17 Al 12 , Mg 3 Bi 2 and Mg 3 Sb 2 phases. The calculation results of heat of formation and cohesive energy indicate that Mg 3 Sb 2 has the strongest alloying ability as well as the highest structural stability. The calculations of thermodynamic properties show that the Gibbs free energy of Mg 17 Al 12 , Mg 3 Bi 2 and Mg 3 Sb 2 change with the elevated temperature, when the temperature is below 423 K, Mg 3 Sb 2 and Mg 3 Bi 2 are more stable than Mg 17 Al 12 phases and Sb or Bi addition to the Mg–Al-based alloys can improve the heat resistance. The densities of states (DOS) of these compounds are given. The elastic parameters are calculated, then the bulk modulus, shear modulus, Young’s modulus and Poisson ratio value are derived. All the results of the analysis indicate that adding Sb or Bi to Mg–Al alloy could improve the ductility and plasticity by forming Mg 3 Sb 2 and Mg 3 Bi 2 phases.

Ping Peng

Papers

Molecular dynamics simulation for cooling rate dependence of solidification microstructures of silver

Formation and magic number characteristics of clusters formed during solidification processes

Thermal stability and elastic properties of Mg3Sb2 and Mg3Bi2 phases from first-principles calculations

A first-principles study on the structural stability of Al2Ca Al4Ca and Mg2Ca phases

First-principles investigation of Mg2Ni phase and high/low temperature Mg2NiH4 complex hydrides