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

The glass transition temperature of PMMA: A molecular dynamics study and comparison of various determination methods

In this report, classic molecular dynamics simulations based on the embedded atom method (EAM) are carried out to study the atomic arrangement in Ni3Al metallic glass. The short-range order (SRO) and medium-range order (MRO) in the rapidly quenched Ni3Al alloy are characterized by the both structural analysis methods; Radial distribution function (RDF) and Voronoi tessellation. From RDF, we found a splitting in the second peak that reflects the atomic packing beyond the nearest neighbors and it is a characteristic of the MRO. Furthermore, the transition from the supercooled state to the glassy state of Ni3Al alloy leads to an increased population of icosahedral-like clusters and their spatial connectivity via vertex-sharing (VS), edge-sharing (ES), face-sharing (FS) and intercross-sharing (IS). Interestingly, by comparing the position ratios of subpeaks in RDF with those of the shared cluster distance to the first peak in RDF, we suggest that the first subpeak originates from the face-sharing clusters while the second one results from the vertex-sharing clusters. However, the edge-sharing is hidden between the two subpeaks due to its smaller fraction in the system.

Atomic packing and medium-range order in Ni 3 Al metallic glass

Structural evaluation and deformation features of interface of joint between nano-crystalline Fe–Ni–Cr alloy and nano-crystalline Ni during creep process

The local atomic structure in aluminium monatomic metallic glass is studied using molecular dynamics simulations combined with the embedded atom method (EAM). We have used a variety of analytical methods to characterise the atomic configurations of our system: the Pair Distribution Function (PDF), the Common Neighbour Analysis (CNA) and the Voronoi Tessellation Analysis. CNA was used to investigate the order change from liquid to amorphous phases, recognising that the amount of icosahedral clusters increases with the decrease of temperature. The Voronoi analysis revealed that the icosahedral-like polyhedral are the predominant ones. It has been observed that the PDF function shows a splitting in the second peak, which cannot be attributed to the only ideal icosahedral polyhedron 〈0, 0, 12, 0〉, but also to the formation of other Voronoi polyhedra 〈0, 1, 10, 2〉 . Further, the PDFs were then integrated giving the cumulative coordination number in order to compute the fractal dimension (df).

Cooling rate dependence and local structure in aluminum monatomic metallic glass

Molecular dynamics simulations have been performed to study the vitrification process and the local atomic structures in Ni monatomic metallic glasses (MG). The interatomic interactions are described by embedded atom method (EAM) potentials. Short-range order (SRO) and medium range order (MRO) in nickel monatomic MG is studied using several structural techniques such as the Radial distribution function (RDF); the Common neighbor analysis method (CNA) and the Voronoi tessellation. As a result, we found that the atomic packing in metallic glasses can be described globally as a superposition of the spherical-periodic order (SPO) and the local translational symmetry (LTS). From CNA method, we found that the majority of icosahedral clusters in Ni monatomic MG are connected together by vertex-sharing (VS), edge-sharing (ES), face-sharing (FS) and intercross-sharing (IS) rather than isolated clusters. These typical cluster connections constitute to the partial RDF of icosahedral atoms except the edge sharing which is hidden between the second and the third subpeaks of RDF. Furthermore, the visualization technologies are applied to follow the formation of clusters during rapid quenching. It is found that the clusters ⟨0,1,10,2⟩ and ⟨0,2,8,4⟩ are more dominant than the full icosahedral polyhedron ⟨0,0,12,0⟩ in our monatomic system. Furthermore, the splitting of the second peak in RDF curve in Ni glass is not only caused by the icosahedral clusters, but also by the Voronoi polyhedrons ⟨0,1,10,2⟩ and ⟨0,2,8,4⟩.

Molecular dynamics study of atomic-level structure in monatomic metallic glass

In this study we examine the structural properties of single-component metallic glasses of aluminum. We use a molecular dynamics simulation based on semi-empirical many-body potential, derived from the embedded atom method (EAM). The radial distribution function (RDF), common neighbors analysis method (CNA), coordination number analysis (CN) and Voronoi tessellation are used to characterize the metal’s local structure during the heating and cooling (quenching). The simulation results reveal that the melting temperature depends on the heating rate. In addition, atomic visualization shows that the structure of aluminum after fast quenching is in a glassy state, confirmed quantitatively by the splitting of the second peak of the radial distribution function, and by the appearance of icosahedral clusters observed via CNA technique. On the other hand, the Wendt-Abraham parameters are calculated to determine the glass transition temperature (T
 g
 ), which depends strongly on the cooling rate; it increases while the cooling rate increases. On the basis of CN analysis and Voronoi tessellation, we demonstrate that the transition from the Al liquid to glassy state is mainly due to the formation of distorted and perfect icosahedral clusters.

Local atomic structures of single-component metallic glasses

In this paper, we investigated the structural properties of metallic glasses (MGs). We emphasized our study on monatomic Al and binary TiAl3 systems. The calculations are performed by using the molecular dynamics (MD) simulation based on semi-empirical many-body potentials derived from the embedded atom method. The structure is analyzed using the radial distribution function (RDF), the common neighbor analysis (CNA) and the coordination numbers (CNs). Our results demonstrated that it is possible to form MGs in both systems upon fast cooling from the liquid state. This is confirmed by the fact that the system energy and/or volume during the cooling stage decrease continuously with a slight change and by atomic scale analysis using the RDF, CNA and CN analyzing techniques. Furthermore, this specific study shows that under the same conditions, the icosahedral structures appeared in TiAl3 are more abundant than in pure Al. Implications of these findings are discussed.

S. Trady

Papers

Molecular dynamics study of atomic-level structure in monatomic metallic glass

Atomic packing and medium-range order in Ni 3 Al metallic glass

Cooling rate dependence and local structure in aluminum monatomic metallic glass

Local atomic structures of single-component metallic glasses

Structural properties of Al and TiAl3 metallic glasses — An embedded atom method study