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

Fluorescent nanodiamond is a new nanomaterial that possesses several useful properties, including good biocompatibility1, excellent photostability1,2 and facile surface functionalizability2,3. Moreover, when excited by a laser, defect centres within the nanodiamond emit photons that are capable of penetrating tissue, making them well suited for biological imaging applications1,2,4. Here, we show that bright fluorescent nanodiamonds can be produced in large quantities by irradiating synthetic diamond nanocrystallites with helium ions. The fluorescence is sufficiently bright and stable to allow three-dimensional tracking of a single particle within the cell by means of either one- or two-photon-excited fluorescence microscopy. The excellent photophysical characteristics are maintained for particles as small as 25 nm, suggesting that fluorescent nanodiamond is an ideal probe for long-term tracking and imaging in vivo, with good temporal and spatial resolution.

Mass production and dynamic imaging of fluorescent nanodiamonds

Mechanical metamaterials associated with stiffness, rigidity and compressibility: a brief review

The incorporation of the desirable properties of graphene into three-dimensional materials remains challenging. Here, the authors report the scalable self-assembly of graphene sheets into spongy materials with very low densities, and near-zero and largely strain-independent Poisson's ratios in all directions.

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Three-dimensionally bonded spongy graphene material with super compressive elasticity and near-zero Poisson’s ratio

Material properties can be tailored through modification of their geometry or architecture. With this concept, a lot of smart materials, metamaterials, and smart structures have been developed. Auxetic materials and structures are a novel class of materials which exhibit an interesting property of negative Poisson's ratio. By virtue of the auxetic behavior, mechanical properties such as fracture toughness, indentation resistance, etc., can be improved. In order to exploit the interesting properties of auxetic materials, several potential applications of auxetic materials have been explored in medical, sports, automobile, defense, etc. Design and modeling of novel auxetic materials and structures is still on the way. Here, the article focuses upon the different aspects of auxetic materials and structures. A comprehensive updated review of auxetic materials, their types and properties, and applications has been presented. This paper also discusses the design and modeling approaches of auxetic structures.

Three Decades of Auxetics Research − Materials with Negative Poisson's Ratio: A Review

In the present paper, we analyze uniaxial deformation of crystals of different systems with negative Poisson’s ratios, known as auxetics. The behavior of auxetic crystals is studied on the basis of extensive knowledge on the experimental values of elastic constants of different crystals, gathered in the well-known Landolt-Bornstein tables. The competition between the anisotropy of crystal structures and the orientation of deformable samples results in the dependence of the elastic characteristics of deformation, such as Young’s modulus and Poisson’s ratio, on the orientation angles. In the special case of a single angle, a large number of auxetics were found among crystals of cubic, hexagonal, rhombohedral, tetragonal, and orthorhombic systems and the character of variations in their response due to changes in orientation was determined.

Auxetic Mechanics of Crystalline Materials

Two-parameter analysis of auxetics among the cubic crystals is proposed. A brief analysis of the equivalence of this two-parameter consideration and other approaches is given. The main result of this paper is the classification of partial auxetics with a single dimensionless complex, which is composed of the crystals elastic compliances. The auxetic surface separates the regions with negative and positive Poisson’s ratio. The character of its changes with change of the dimensionless complex is determined. The critical value of the complex where a topological rearrangement of the auxetic surface occurs is obtained. The distribution of partial auxetics in zones with different values of the dimensionless complex was found. The sign of another dimensionless parameter influences the location of a region with negative Poisson’s ratio relative to the auxetic surface. View of the auxetic boundary for a cubic crystal when the dimensionless elastic parameter is close to the critical value Pc � 0:745.

Classification of cubic auxetics

Negative Poisson’s ratio for cubic crystals and nano/microtubes

Diamond-like carbon nanostructures with cubic anisotropy made by joining fullerene-like molecules of different types via valence bonds are studied by means of molecular dynamics simulations. The considered structures are interesting because they include both - and -hybridized carbon atoms, which lead to their distinct properties compared to the structures with one type of hybridization. Seven diamond-like carbon phases having different shapes of structural units and/or different ways of their connection are studied in the present work. For the relaxed equilibrium structures, the engineering elastic constants (Poisson's ratio, Young's modulus, and shear modulus) are calculated as the functions of the crystal orientation angles. Extreme values of the elastic constants are reported. It is shown that two of the considered diamond-like structures have negative Poisson's ratio and can be regarded as the partial auxetics. According to the results of the present study, elastic properties of the bulk diamond-like carbon structures can vary considerably depending on their structure.







Diamond-like carbon nanostructures

Equilibrium diamond-like carbon nanostructures with cubic anisotropy: Elastic properties

Mechanical response of the carbon nanotube bundle to uniaxial and biaxial lateral compression followed by unloading is modeled under plane strain conditions. The chain model with a reduced number of degrees of freedom is employed with high efficiency. During loading, two critical values of strain are detected. Firstly, period doubling is observed as a result of the second order phase transition, and at higher compressive strain, the first order phase transition takes place when carbon nanotubes start to collapse. The loading-unloading stress-strain curves exhibit a hysteresis loop and, upon unloading, the structure returns to its initial form with no residual strain. This behavior of the nanotube bundle can be employed for the design of an elastic damper.

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Dmitry S. Lisovenko

Papers

Auxetic Mechanics of Crystalline Materials

Classification of cubic auxetics

Negative Poisson’s ratio for cubic crystals and nano/microtubes

Equilibrium diamond-like carbon nanostructures with cubic anisotropy: Elastic properties

Elastic damper based on the carbon nanotube bundle