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

https://dc.engconfintl.org/cgi/viewcontent.cgi?article=1032&context=superalloys_ii

A critical review of high entropy alloys and related concepts

We describe a global optimization technique using “basin-hopping” in which the potential energy surface is transformed into a collection of interpenetrating staircases. This method has been designed to exploit the features that recent work suggests must be present in an energy landscape for efficient relaxation to the global minimum. The transformation associates any point in configuration space with the local minimum obtained by a geometry optimization started from that point, effectively removing transition state regions from the problem. However, unlike other methods based upon hypersurface deformation, this transformation does not change the global minimum. The lowest known structures are located for all Lennard-Jones clusters up to 110 atoms, including a number that have never been found before in unbiased searches.

/pdf/global-optimization-by-basin-hopping-and-the-lowest-energy-2s1wxtsj18.pdf

Global Optimization by Basin-Hopping and the Lowest Energy Structures of Lennard-Jones Clusters Containing up to 110 Atoms

Alloying has long been used to confer desirable properties to materials. Typically, it involves the addition of relatively small amounts of secondary elements to a primary element. For the past decade and a half, however, a new alloying strategy that involves the combination of multiple principal elements in high concentrations to create new materials called high-entropy alloys has been in vogue. The multi-dimensional compositional space that can be tackled with this approach is practically limitless, and only tiny regions have been investigated so far. Nevertheless, a few high-entropy alloys have already been shown to possess exceptional properties, exceeding those of conventional alloys, and other outstanding high-entropy alloys are likely to be discovered in the future. Here, we review recent progress in understanding the salient features of high-entropy alloys. Model alloys whose behaviour has been carefully investigated are highlighted and their fundamental properties and underlying elementary mechanisms discussed. We also address the vast compositional space that remains to be explored and outline fruitful ways to identify regions within this space where high-entropy alloys with potentially interesting properties may be lurking. High-entropy alloys have greatly expanded the compositional space for alloy design. In this Review, the authors discuss model high-entropy alloys with interesting properties, the physical mechanisms responsible for their behaviour and fruitful ways to probe and discover new materials in the vast compositional space that remains to be explored.

High-entropy alloys

The structural properties of free nanoclusters are reviewed. Special attention is paid to the interplay of energetic, thermodynamic, and kinetic factors in the explanation of cluster structures that are actually observed in experiments. The review starts with a brief summary of the experimental methods for the production of free nanoclusters and then considers theoretical and simulation issues, always discussed in close connection with the experimental results. The energetic properties are treated first, along with methods for modeling elementary constituent interactions and for global optimization on the cluster potential-energy surface. After that, a section on cluster thermodynamics follows. The discussion includes the analysis of solid-solid structural transitions and of melting, with its size dependence. The last section is devoted to the growth kinetics of free nanoclusters and treats the growth of isolated clusters and their coalescence. Several specific systems are analyzed.

/pdf/structural-properties-of-nanoclusters-energetic-56vj4t3ucw.pdf

Structural properties of nanoclusters: Energetic, thermodynamic, and kinetic effects

We have performed simulations of coexisting liquid and solid phases of aluminum as an efficient way of mapping out the coexistence line. This technique is convenient, as it does not require complicated free-energy calculations for the different phases, but simply allows the system to equilibrate to a co-existence point. By altering the simulation volume and/or energy, a new coexistence point is found. The calculated method temperature is lower than previous results for the identical model; we suspect that this difference is due to the difficulty of calculating the free energy of the liquid phase, leading to inaccuracies in the previous work. A thorough examination of several different system sizes, from 1024 to 65 536 particles, indicates that the results are only weakly dependent upon the system size.

/pdf/melting-line-of-aluminum-from-simulations-of-coexisting-17dp2dwzvm.pdf

Melting line of aluminum from simulations of coexisting phases.

High-entropy alloys are a new class of materials that have been shown to be strong, ductile, and corrosion-resistant. Ensembles of viable alloys are systematically isolated using density-functional theory.

https://link.aps.org/pdf/10.1103/PhysRevX.5.011041

Criteria for Predicting the Formation of Single-Phase High-Entropy Alloys

Stoichiometric intermetallic compounds have always been touted for their attractive chemical, physical, electrical, magnetic and mechanical properties, but few practical uses have materialized because they are brittle at room temperature1,2,3,4. Here we report on a large family of fully ordered, stoichiometric binary rare-earth intermetallic compounds with high ductility at room temperature. Although conventional wisdom calls for special conditions, such as non-stoichiometry, metastable disorder or doping to achieve some ductility in intermetallic compounds at room temperature, none of these is required in these unique B2 rare-earth compounds. Ab initio calculations of YAg, YCu and NiAl crystal defect energies support the observed deformation modes of these intermetallics.

A family of ductile intermetallic compounds.

Structural optimization of Lennard-Jones clusters by a genetic algorithm

We have performed large-scale molecular dynamics simulations of coexisting solid and liquid phases using 4e(σ/r)n interactions for n=9 and n=12, and for Lennard-Jones systems, in order to calculate the equilibrium melting curve. The coexisting systems evolve rapidly toward the melting temperature. The P–T melting curves agree well with previous calculations, as do the other bulk phase properties. The melting curve for the Lennard-Jones system, evaluated using various truncations of the potential, converges rapidly as a function of the potential cutoff, indicating that long-range corrections to the free energies of the solid and liquid phases very nearly cancel. This approach provides an alternative to traditional methods of calculating melting curves.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1371&context=ameslab_pubs

James R. Morris

Papers

Melting line of aluminum from simulations of coexisting phases.

Criteria for Predicting the Formation of Single-Phase High-Entropy Alloys

A family of ductile intermetallic compounds.

Structural optimization of Lennard-Jones clusters by a genetic algorithm

The melting lines of model systems calculated from coexistence simulations