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We present results of the atomic and electronic structure of icosahedral (Formula presented), Ge and Sn) clusters using the ab initio molecular-dynamics method within the local density functional theory. Substitutional doping of a (Formula presented) cluster by a tetravalent atom leads to a substantial gain in energy in all the cases studied. Tin is found to have a lower energy at a vertex site in contrast to the central site for Si and Ge, leading to surface segregation of Sn in these clusters. Also in the case of a (Formula presented) cluster, Si occupies the central site of a capped icosahedral structure. These results when interpreted in terms of the interaction of closed shell clusters with atoms leads to a relatively strong interaction of (Formula presented) with (Formula presented) as compared to the weak interaction of rare gas atoms with other elements. {\textcopyright} 1998 The American Physical Society.

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Physical Review B - Condensed Matter and Materials Physics

Ab initio molecular-dynamics studies of doped magic clusters and their interaction with atoms

Frontiers in Materials Modelling and Design

Materials Modelling and Design: An Introduction

"These proceedings bring together state-of-the-art ab-initio and macroscopic, "coarse-grained" approaches for understanding technologically important problems in materials science. Contributions by experts present our current understanding of alloys, defects and deformation behaviour, surface phenomena and the properties of small clusters and nano-materials. The use of simulational tools such as the Car-Parinello method, Monte Carlo method, classical molecular dynamics and continuum finite element methods to study materials structure and behaviour is emphasized. Fundamental understanding obtained from such studies should help in designing novel materials with desired properties and in improving materials performance."--BOOK JACKET.

Frontiers in materials modelling and design : proceedings of the Conference on Frontiers in Materials Modelling and Design, Kalpakkam, 20-23 August 1996

First principles calculations and simulations based on interatomic potentials together with experimental studies of abundance spectrum suggest icosahedral structures to be common for some magic clusters of diverse systems such as rare gases, metals, covalently bonded systems and water. Close packing models obtained from pair potentials are shown to be good representations of the structure of rare gas clusters. However, the electronic structure is found to play the important role in the atomic structure and related properties of other clusters. Results of recent studies of fullerenes, their derivatives as well as some large icosahedral metal clusters containing several thousand atoms are also presented. Further results on doped icosahedral clusters are discussed which hold promise for the development of new materials and for understanding the occurrence of icosahedral order in several aluminum alloys.

Progress in Crystal Growth and Characterization of Materials

Icosahedral symmetry in clusters

A simple algorithm for solving the hollow-fiber bioreactor design equations has been presented. This algorithm is quite general and is applicable to any nonlinear reaction occurring in the reactor spongy matrix. This can also be applied to other reactor types having similar configurations, like the wall-coated enzyme reactors.

Chemical Engineering Journal

An efficient algorithm for solving hollow-fiber bioreactor design equations

We present results of our ab initio molecular dynamics simulations on the atomic and electronic structure of clusters of divalent metals, aluminum and antimony, which exhibit a range of bonding characteristics e.g. non-metal-metal transition, metallic and covalent respectively. Results of these studies have been used to develop icosahcdral Al12X (X = C, Si and Ge) superatoms with 40 valence electrons which correspond to a filled electronic shell. It is found that the doping leads to a large gain in the binding energy as compared to Al13, suggesting this to be a novel way of developing species for cluster assembled materials. Further studies of adsorption of Li, Si and Cl atoms on Al7 and Al13 clusters show marked variation in the adsorption behaviour of clusters as a function of size and the adsorbate. Silicon reconstructs both the clusters and induces covalency in Al-Al bonds. We discuss the adsorption behaviour in terms of the superatom-atom interactions.

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Bulletin of Materials Science

Ab initio molecular dynamics studies of metal clusters

We have studied structural properties of amorphous hydrogenated silicon using ab initio molecular dynamics simulations. A sample was generated by simulated annealing using periodic boundary conditions with a supercell containing 64 silicon and 8 hydrogen atoms. The radial pair distribution functions for Si-Si, Si-H and H-H have been studied at 300 K and are found to be in good agreement with experimental data. Our results show that hydrogen saturates the dangling bonds and reduces bond strain. We also report existence of Si-H-Si bridge sites which are likely to play an important role in understanding the light induced metastability in this material.

Structural properties of amorphous hydrogenated silicon using ab initio molecular dynamics simulations

Shifting an oscillatory or a chaotic trajectory to the unstable steady state of a nonlinear system in the presence of stochastic or deterministic load disturbances continues to he a nontrivial task. In the present work, two effective strategies for such control needs are presented. The control laws employed do not contain the process model parameters explicitly. The suggested strategies are demonstrated on two simulated nonlinear reaction systems exhibiting multi-stationarity, limit cycle oscillations, and chaos. Shifting an oscillatory or a chaotic trajectory to the unstable steady state of a nonlinear system in the presence of stochastic or deterministic load disturbances continues to be a nontrivial task. In the present work, two effective strategies for such control needs are presented. The control laws employed do not contain the process model parameters explicitly. The suggested strategies are demonstrated on two simulated nonlinear reaction systems exhibiting multi-stationarity, limit cycle oscillations, and chaos.

On control of nonlinear system dynamics at unstable steady state

A detailed stochastic analysis of the Brusselator scheme has been performed to bring out the effect of external noise on the system. The diffusion parameter [formula presented] was taken to be fluctuated by external noise and a complete solution diagram with the composition variables X and Y has been generated. These noise-induced transitions reveal that this system attains a stable state not described by its deterministic analysis. The stochastic analyses also reveal that the structural stability of such systems is disturbed even for a slight external perturbation in the bifurcation parameter, and in a certain range of noise intensity (σ) and correlation time (τ) some different spatial and temporal structures arise. © 1997 The American Physical Society.

Physical Review E - Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics

Analysis of bifurcation patterns in reaction-diffusion systems: Effect of external noise on the Brusselator model

With the discovery of solid C60, efforts are being made to develop new clusters and molecules which could be assembled to form new materials. Here we present some recent developments in this direction and discuss bonding in such materials.