A Finnis–Sinclair-type many-body potential and a combined Lennard-Jones (two-body) plus Axilrod–Teller (three-body) potential are employed to study the structural properties of model clusters in the size range 13 to 309 atoms. For the first potential, parameter values for the fcc transition metals as proposed by Sutton and Chen are used, while for the latter potential a least-squares fitting routine is employed to fit the parameters to bulk data. For suitable parameter ranges, the relative energies of optimized decahedral, icosahedral, and face-centered-cubic (fcc) cuboctahedral clusters are compared in the light of experimental observations for these metals. Variation of the parameters in the Finnis–Sinclair potential, which govern the range of the repulsive two-body term and the attractive many-body term, influences the size at which the icosahedra and fcc cuboctahedra cross over in energy. Similarly, an increase in the three-body contribution in the LJ+AT clusters leads to a decrease in the crossover size from several thousand to a few hundred atoms. For appropriate parameter ranges, a convex curvature of the facets as well as a contraction of the clusters is observed for both potentials. These effects are compared with experimental observations for the fcc transition metals copper, silver, gold, nickel, palladium, and platinum.

No abstract

No abstract

C hemical reactions are usually thought of in terms of the breaking and forming of strong covalent bonds. However, weaker interactions, such as hydrogen bonds, salt bridges, and solvation forces, can add up and play just as strong a role, determining, for example, the properties of liquids, the solubility of solids, and the organization of molecules in membranes. Nature has exploited these interactions in biorecognition and biomolecular organization for billions of years. Learning from biology, chemists are now developing highly complex chemical systems from components that interact via noncovalent intermolecular forces. Such forces were exploited long before the terms "supramolecular chemistry" and "self-assembly" were introduced. The difference is that in supramolecular chemistry, molecules are designed and synthesized to interact specifically with other molecules or to form larger aggregates. With just a few building blocks, strands of nucleic acids allow huge amounts of information to be sorted, retrieved, and processed via weak hydrogen bonds. Lehn (p. 2400) discusses how supramolecular chemistry has implemented these principles of molecular information in chemistry. Increasingly, the ability of supramolecular assemblies to adapt and evolve is exploited. Reinhoudt and Crego-Calama (p. 2403) define a supramolecule as "a collection of atoms held together by covalent and noncovalent bonds." Noncovalent and covalent synthesis both aim to introduce specific connectivities between atoms, but supramolecules may be highly dynamic, allowing for error correction and chemical evolution. A News story (p. 2396) discusses how chemists are using noncovalent bonds to revolutionize everything from polymers to materials implanted in the body. Whitesides and Grzybowski (p. 2418) define self-assembly as a process that involves preexisting components, is reversible, and can be controlled by proper design of the components. They consider the possible applications of self-assembly from crystallization to robotics to nanotechnology. Knowledge of what drives and controls self-assembly is necessary for understanding life itself, as illustrated by the report by Sumper (p. 2430) on the role of selfassembly in the formation of diatom shells. Organic crystals and liquid crystals represent two areas in which weak interactions have long been exploited. Hollingsworth (p. 2410) discusses the progress made in using crystal packing to create porous networks, which can be exploited in gas separation and storage, and the use of hostguest chemistry to change the ordering and symmetry of a crystal, which can lead to materials with nonlinear optical properties. Kato (p. 2414) reviews how self-assembly and phase segregation in liquid crystalline systems can be used in drug delivery, batteries, and electro-optical devices. Ikkala and ten Brinke (p. 2407) discuss how the self-assembly of polymeric supramolecules can be used to produce hierarchically structured functional materials. By controlling the structure and properties of the material at differe...

Clusters of 13-150 Lennard-Jones atoms have been investigated, with attention given to clusters at both 'magic' and intermediate nuclearities. Icosahedral, decahedral and face-centred-cubic structures are compared in terms of their relative energies and the topology of the potential-energy surfaces. In particular, both gradient and second-derivative optimisation schemes are employed, and all structures are characterised in terms of their force constants. W e find a number of interesting relationships between the three principal structural morphologies. Different capping routes are also compared to investigate possible growth schemes for t h e clusters of intermediate size. W e find that t h e presence or absence of particular capping or surface atoms can be crucial in determining structural stability, showing that even this simple model potential can provide insights into the chemistry of small metal particles, as observed from experiment.

No abstract

No abstract

No abstract