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

Although gold is the subject of one of the most ancient themes of investigation in science, its renaissance now leads to an exponentially increasing number of publications, especially in the context of emerging nanoscience and nanotechnology with nanoparticles and self-assembled monolayers (SAMs). We will limit the present review to gold nanoparticles (AuNPs), also called gold colloids. AuNPs are the most stable metal nanoparticles, and they present fascinating aspects such as their assembly of multiple types involving materials science, the behavior of the individual particles, size-related electronic, magnetic and optical properties (quantum size effect), and their applications to catalysis and biology. Their promises are in these fields as well as in the bottom-up approach of nanotechnology, and they will be key materials and building block in the 21st century. Whereas the extraction of gold started in the 5th millennium B.C. near Varna (Bulgaria) and reached 10 tons per year in Egypt around 1200-1300 B.C. when the marvelous statue of Touthankamon was constructed, it is probable that “soluble” gold appeared around the 5th or 4th century B.C. in Egypt and China. In antiquity, materials were used in an ecological sense for both aesthetic and curative purposes. Colloidal gold was used to make ruby glass 293 Chem. Rev. 2004, 104, 293−346

Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology.

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Self-assembled monolayers of thiolates on metals as a form of nanotechnology.

Fundamentals of Plasmonics.- Electromagnetics of Metals.- Surface Plasmon Polaritons at Metal / Insulator Interfaces.- Excitation of Surface Plasmon Polaritons at Planar Interfaces.- Imaging Surface Plasmon Polariton Propagation.- Localized Surface Plasmons.- Electromagnetic Surface Modes at Low Frequencies.- Applications.- Plasmon Waveguides.- Transmission of Radiation Through Apertures and Films.- Enhancement of Emissive Processes and Nonlinearities.- Spectroscopy and Sensing.- Metamaterials and Imaging with Surface Plasmon Polaritons.- Concluding Remarks.

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Plasmonics: Fundamentals and Applications

The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

We develop a chain model of an electrorheological fluid in steady and oscillatory shear. This model is based on a balance of hydrodynamic and electrostatic forces and focuses on the mechanical stability of chains. With this model we compute the shape and orientation of an unconfined dipolar chain in steady shear as a function of the shear rate and electric field, and show that this leads to the expected shear‐thinning viscosity. We then demonstrate that chains confined by electrodes are unstable in some Mason number regimes. The chain model is extended to the case of oscillatory shear by including a kinetic equation for the aggregation and fragmentation of chains. The resultant chain dynamics is found to be strongly nonlinear, as is the rheology. Finally, we consider the effect of local field corrections and long range interactions on the bare dipolar interaction force and derive a self‐consistent pair interaction force that demonstrates that the maximum orientation of a chain in shear depends strongly on the dielectric contrast.

Chain model of electrorheology

We have investigated the formation of 2-D and 3-D superlattices of Au nanoclusters synthesized in nonionic inverse micelles, and capped with alkanethiol ligands, with alkane chains ranging from C6 to C18. The thiols are found to play a significant role in the ripening of these nanoclusters, and in the formation of superlattices. Image processing techniques were developed to reliably extract from transmission electron micrographs (TEMs) the particle size distribution, and information about the superlattice domains and their boundaries. The latter permits us to compute the intradomain vector pair correlation function, from which we can accurately determine the lattice spacing and the coherent domain size. From these data the gap between the particles in the coherent domains can be determined as a function of the thiol chain length. It is found that as the thiol chain length increases, the nanoclusters become more polydisperse and larger, and the gaps between particles within superlattice domains increase. A...

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Control of the Interparticle Spacing in Gold Nanoparticle Superlattices

In this review we discuss recent research on driving self-assembly of magnetic particle suspensions subjected to alternating magnetic fields. The variety of structures and effects that can be induced in such systems is remarkably broad due to the large number of variables involved. The alternating field can be uniaxial, biaxial or triaxial, the particles can be spherical or anisometric, and the suspension can be dispersed throughout a volume or confined to a soft interface. In the simplest case the field drives the static or quasistatic assembly of unusual particle structures, such as sheets, networks and open-cell foams. More complex, emergent collective behaviors evolve in systems that can follow the time-dependent field vector. In these cases energy is continuously injected into the system and striking flow patterns and structures can arise. In fluid volumes these include the formation of advection and vortex lattices. At air–liquid and liquid–liquid interfaces striking dynamic particle assemblies emerge due to the particle-mediated coupling of the applied field to surface excitations. These out-of-equilibrium interface assemblies exhibit a number of remarkable phenomena, including self-propulsion and surface mixing. In addition to discussing various methods of driven self-assembly in magnetic suspensions, some of the remarkable properties of these novel materials are described.

Driving self-assembly and emergent dynamics in colloidal suspensions by time-dependent magnetic fields.

A scaling form for the pair correlation function of random fractals is combined with the scaling form of the percolation distribution of cluster sizes to obtain expressions for the static structure factor S(q) and first cumulant ${K}_{1}$ of the dynamic structure factor S(q,t) at the gel point. We find S(q)\ensuremath{\sim}${q}^{\mathrm{\ensuremath{-}}\ensuremath{\mu}}$ and ${K}_{1}$\ensuremath{\sim}${q}^{3}$ with \ensuremath{\mu}=D(3-\ensuremath{\tau}), where D is the fractal dimension and \ensuremath{\tau} is the exponent for the distribution of cluster sizes. Special forms are suggested for the scaling functions to obtain results for the nonscaling regime.

Static and dynamic scattering from fractals

We present experimental and theoretical results on the shear thinning of electrorheological fluids. Rheological measurements on a model fluid consisting of monodisperse silica spheres immersed in a dielectric liquid show a power-law dependence \ensuremath{\mu}\ensuremath{\propto}\ensuremath{\gamma}${\mathrm{\ifmmode \dot{}\else \.{}\fi{}}}^{\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\Delta}}}$ of the apparent viscosity \ensuremath{\mu} on the strain rate \ensuremath{\gamma}\ifmmode \dot{}\else \.{}\fi{}, with \ensuremath{\Delta} in the range \ensuremath{\Delta}=0.68--0.93. We present a theoretical treatment of steady-state cluster formation in applied electric and shear fields, which correctly predicts the observed power-law shear thinning albeit with an exponent \ensuremath{\Delta}=2/3. We observe no true yield stress in this material.

James E. Martin

Papers

Chain model of electrorheology

Control of the Interparticle Spacing in Gold Nanoparticle Superlattices

Driving self-assembly and emergent dynamics in colloidal suspensions by time-dependent magnetic fields.

Static and dynamic scattering from fractals

Rheology of electrorheological fluids.