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

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

BACKGROUND With a rising global population, increasing energy demands, and impending climate change, major concerns have been raised over the security of our energy future. Developing sustainable, fossil-free pathways to produce fuels and chemicals of global importance could play a major role in reducing carbon dioxide emissions while providing the feedstocks needed to make the products we use on a daily basis. One prospective goal is to develop electrochemical conversion processes that can convert molecules in the atmosphere (e.g., water, carbon dioxide, and nitrogen) into higher-value products (e.g., hydrogen, hydrocarbons, oxygenates, and ammonia) by coupling to renewable energy. Electrocatalysts play a key role in these energy conversion technologies because they increase the rate, efficiency, and selectivity of the chemical transformations involved. Today’s electrocatalysts, however, are inadequate. The grand challenge is to develop advanced electrocatalysts with the enhanced performance needed to enable widespread penetration of clean energy technologies. ADVANCES Over the past decade, substantial progress has been made in understanding several key electrochemical transformations, particularly those that involve water, hydrogen, and oxygen. The combination of theoretical and experimental studies working in concert has proven to be a successful strategy in this respect, yielding a framework to understand catalytic trends that can ultimately provide rational guidance toward the development of improved catalysts. Catalyst design strategies that aim to increase the number of active sites and/or increase the intrinsic activity of each active site have been successfully developed. The field of hydrogen evolution, for example, has seen important breakthroughs over the years in the development of highly active non–precious metal catalysts in acid. Notable advancements have also been made in the design of oxygen reduction and evolution catalysts, although there remains substantial room for improvement. The combination of theory and experiment elucidates the remaining challenges in developing further improved catalysts, often involving scaling relations among reactive intermediates. This understanding serves as an initial platform to design strategies to circumvent technical obstacles, opening up opportunities and approaches to develop higher-performance electrocatalysts for a wide range of reactions. OUTLOOK A systematic framework of combining theory and experiment in electrocatalysis helps to uncover broader governing principles that can be used to understand a wide variety of electrochemical transformations. These principles can be applied to other emerging and promising clean energy reactions, including hydrogen peroxide production, carbon dioxide reduction, and nitrogen reduction, among others. Although current paradigms for catalyst development have been helpful to date, a number of challenges need to be successfully addressed in order to achieve major breakthroughs. One important frontier, for example, is the development of both experimental and computational methods that can rapidly elucidate reaction mechanisms on broad classes of materials and in a wide range of operating conditions (e.g., pH, solvent, electrolyte). Such efforts would build on current frameworks for understanding catalysis to provide the deeper insights needed to fine-tune catalyst properties in an optimal manner. The long-term goal is to continue improving the activity and selectivity of these catalysts in order to realize the prospects of using renewable energy to provide the fuels and chemicals that we need for a sustainable energy future.

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Combining theory and experiment in electrocatalysis: Insights into materials design

Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.

Shape‐Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?

This paper describes a facile method that generates dimers of Ag nanospheres by etching Ag nanocubes with Fe(NO3)3 in ethanol with the assistance of poly(vinyl pyrrolidone) (PVP). During the etching process, the corners and edges of the Ag nanocubes were truncated off to generate spherical particles, accompanied by dimerization as a result of reduction in colloidal stability due to the addition of ionic species. Both ethanol and PVP play an important role in the etching and dimerization processes. By starting with Ag nanocubes of different sizes, we obtained well-defined dimers of Ag spheres 40, 63, and 80 nm in diameter with percentages of dimerization >60%. Since this approach can be used to fabricate dimers of Ag nanospheres with a range of different sizes, it allows for a systematic study of the hot-spot phenomenon in SERS. By correlating with SEM imaging, we measured the SERS enhancement factors for individual dimers from the three different samples, and an average value of 3.9×107, 9.3×107, and 1.7×108 was obtained, respectively.

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Etching and Dimerization: A Simple and Versatile Route to Dimers of Silver Nanospheres with a Range of Sizes

There is a growing interest in controlling the synthesis of colloidal metal nanocrystals and thus tailoring their properties toward various applications. In this context, choosing an appropriate combination of reagents (e.g., salt precursor, reductant, capping agent, and stabilizer) plays a pivotal role in enabling the synthesis of metal nanocrystals with diversified sizes, shapes, and structures. Here we present a comprehensive review that highlights one of the key reagents for the synthesis of metal nanocrystals via chemical reduction: the reductants. We start with a brief introduction to the compounds commonly employed as reductants in the colloidal synthesis of metal nanocrystals by showing their oxidation half-reactions and the corresponding oxidation potentials. Then we offer specific examples pertaining to the controlled synthesis of metal nanocrystals, followed by some fundamental aspects covering the general mechanisms of metal ion reduction based on the Marcus Theory. Afterwards, we present a case-by-case discussion on a wide variety of reductants, including their major properties, reduction mechanisms, and additional effects on the final products. We illustrate these aspects by selecting key examples from the literature and paying close attention to the underlying mechanism in each case. At the end, we conclude by summarizing the highlights of the review and providing some perspectives on future directions.

Synthesis of Colloidal Metal Nanocrystals: A Comprehensive Review on the Reductants.

Twin‐Induced Growth of Palladium–Platinum Alloy Nanocrystals

We have investigated the surface-enhanced Raman scattering (SERS) of chemically prepared single-crystal nanocubes and nanospheres of Ag with three different molecules to quantitatively understand the effect of sharp features on the SERS enhancement factor. Both experimental measurements and theoretical calculations confirmed a higher SERS activity for the nanocubes as a result of sharp features on their surfaces. We also found major discrepancies between the measured SERS intensities and those predicted from the electromagnetic mechanism. Through analysis of SERS bands, we concluded that sharp features on the Ag nanocubes could greatly increase the contribution of the chemical enhancement to the SERS intensity.

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Surface-Enhanced Raman Scattering: Comparison of Three Different Molecules on Single-Crystal Nanocubes and Nanospheres of Silver†

This communication describes an investigation of the galvanic replacement reaction between single-crystal Pd nanorods and AuCl4- ions. We found that the Au atoms resulting from the galvanic replacement reaction did not coat the entire surface of a Pd nanorod to generate a core-sheath or hollow nanostructure. In the earlier stages of the reaction, Au deposition was localized to both ends of the Pd nanorod. Then, a transition from two-end to one-end growth was observed, producing a new type of hybrid nanostructure in the shape of a tadpole consisting of a Au head and a Pd tail. Beyond this point, the Au served as nucleation sites for further Au deposition until the hybrid nanostructure was dismantled into round Au nanoparticles and smaller/shorter Pd fragments.

Pedro H. C. Camargo

Papers

Etching and Dimerization: A Simple and Versatile Route to Dimers of Silver Nanospheres with a Range of Sizes

Synthesis of Colloidal Metal Nanocrystals: A Comprehensive Review on the Reductants.

Twin‐Induced Growth of Palladium–Platinum Alloy Nanocrystals

Surface-Enhanced Raman Scattering: Comparison of Three Different Molecules on Single-Crystal Nanocubes and Nanospheres of Silver†

Facile synthesis of tadpole-like nanostructures consisting of Au heads and Pd tails.