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

Defects are often introduced to increase the stiffness of three-dimensional materials. Evidence now suggests that the elastic modulus of two-dimensional graphene sheets can also be increased by controlled defect creation.

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Increasing the elastic modulus of graphene by controlled defect creation

This Perspective presents the development in living anionic polymerization since 1990. The main subjects involve the following concerns of living anionic polymerization: functional styrene derivatives, new monomers and promising additives, the regio- and stereoselective polymerization, special polymers having rigid-rod-like or helical conformations, the synthesis of complex branched polymers composed of comblike segments via living anionic poly(macromonomer)s, and the precise synthesis of macromolecular architectures including multiblock polymers, exact graft polymers, multicomponent μ-star polymers, dendrimer-like star-branched polymers, and hyperbranched polymers by the recently developed methodologies and a new all-around iterative methodology. Throughout the Perspective, attention has focused on recent advances both in the precisely controlled functional polymer syntheses and in various structurally elaborate complex macromolecular architectures. Such advances have significantly expanded the range of ...

Advances in Living Anionic Polymerization: From Functional Monomers, Polymerization Systems, to Macromolecular Architectures

Recent progress of simulations/modeling at the atomic level has led to a better understanding of the mechanical behaviors of graphene, which include the linear elastic modulus E, the nonlinear elastic modulus D, the Poisson’s ratio ν, the intrinsic strength σint and the corresponding strain eint as well as the ultimate strain emax (the fracture strain beyond which the graphene lattice will be unstable). Due to the two-dimensional geometric characteristic, the in-plane tensile response and the free-standing indentation response of graphene are the focal points in this review. The studies are based on multiscale levels: including quantum mechanical and classical molecular dynamics simulations, and parallel continuum models. The numerical studies offer useful links between scientific research with engineering application, which may help to fulfill graphene potential applications such as nano sensors, nanotransistors, and other nanodevices.

https://www.mdpi.com/2073-4360/6/9/2404/pdf

Atomistic Studies of Mechanical Properties of Graphene

New materials with distinctive properties are arising and attracting the scientific community at regular intervals. Stiffness and strength are the important factors in determining stability and lifetime of any technological devices, but defects which are inevitable at the time of production can alter the structural properties of any engineering materials. Developing graphene with specific structural properties depends upon controlling these defects, either by removing or deliberately engineering atomic structure to gain or tailoring specific properties. In this article, a comprehensive review of defective graphene sheets with respect to its mechanical and thermal properties are presented and examined.

Effect of Point and Line Defects on Mechanical and Thermal Properties of Graphene: A Review

The Young's modulus of graphene with various rectangular and circular vacancy defects is investigated by molecular dynamics simulation. By comparing with the results calculated from an effective spring model, it is demonstrated that the Young's modulus of graphene is largely correlated to the size of vacancy defects perpendicular to the stretching direction. And a linear reduction of Young's modulus with the increasing concentration of mono-atomic-vacancy defects (i.e., the slope of −0.03) is also observed. The fracture behavior of graphene, including the fracture strength, crack initiation and propagation are then studied by the molecular dynamics simulation, the effective spring model, and the quantized fracture mechanics. The blunting effect of vacancy edges is demonstrated, and the characterized crack tip radius of 4.44 A is observed.

Jian Zhu

Papers

Effect of Vacancy Defects on the Young's Modulus and Fracture Strength of Graphene: A Molecular Dynamics Study

Easy synthesis of dendrimer-like polymers through a divergent iterative “end-grafting” method