Close-packing transitions in clusters of Lennard-Jones spheres

The structures of clusters of spherical and homogeneous particles are investigated using a combination of global optimization methods. The pairwise potential between particles is integrated exactly from elementary Lennard-Jones interactions, and the use of reduced units allows us to get insight into the effects of the particle diameter. As the diameter increases, the potential becomes very sharp, and the cluster structure generally changes from icosahedral (small radius) to close-packed cubic (large radius), possibly through intermediate decahedral shapes. The results are interpreted in terms of the effective range of the potential

Crystallization of ion clouds in octupole traps: structural transitions, core melting, and scaling laws

International audienceThe stable structures and melting properties of ion clouds in isotropic octupole traps are investigated using a combination of semi-analytical and numerical models, with a particular emphasis at finite size scaling effects. Small-size clouds are found to be hollow and arranged in shells corresponding approximately to the solutions of the Thomson problem. The shell structure is lost in clusters containing more than a few thousands of ions, the inner parts of the cloud becoming soft and amorphous. While melting is triggered in the core shells, the melting temperature unexpectedly follows the rule expected for three-dimensional dense particles, with a depression scaling linearly with the inverse radius

Theoretical study of the hydrated Gd3+ ion: Structure, dynamics, and charge transfer

The dynamical processes taking place in the first coordination shells of the gadolinium (III) ion are important for improving the contrast agent efficiency in magnetic-resonance imaging. An extensive study of the gadolinium (III) ion solvated by a water cluster is reported, based on molecular dynamics simulations. The AMOEBA force field [P. Y. Ren and J. W. Ponder, J. Phys. Chem. B 107, 5933 (2003)] that includes many-body polarization effects is used to describe the interactions among water molecules, and is extended here to treat the interactions between them and the gadolinium ion. In this purpose accurate ab initio calculations have been performed on Gd3+-H2O for extracting the relevant parameters. Structural data of the first two coordination shells and some dynamical properties such as the water exchange rate between the first and second coordination shells are compared to available experimental results. We also investigate the charge transfer processes between the ion and its solvent, using a fluctuating charges model fitted to reproduce electronic structure calculations on [Gd(H2O)n]3+ complexes, with n ranging from 1 to 8. Charge transfer is seen to be significant (about one electron) and correlated with the instantaneous coordination of the ion

FRAGMENTATION DYNAMICS OF IONIZED RARE-GAS CLUSTERS: NEW ACHIEVEMENTS

D. Bonhommeau, N. Halberstadt and U. Buck, Int. Rev. Phys. Chem. 26F. Calvo, D. Bonhommeau and P. Parneix, Phys. Rev. Lett. 99Author Institution: Department of Chemistry, University of Minnesota, 207 Pleasant Street S.E., Minneapolis, MN 55455-0431, USA; LCAR-IRSAMC, Universite Paul Sabatier and CNRS, 118 route de Narbonne, F-31062 Toulouse CEDEX 09, France; Max-Planck Institut fur Dynamik und Selbstoganisation, Busenstr. 10, D-37073 Gottingen, Germany; LCPQ-IRSAMC, Universite Paul Sabatier, 118 route de Narbonne, F-31062 Toulouse, France; Laboratoire de Photophysique Moleculaire, CNRS Bat. 210, Universite Paris-Sud, F-91405 Orsay, FranceThe fragmentation of rare-gas clusters Rg$_n$ ($2\le n\le 14$ and Rg = Ne, Ar and Kr) upon electron-impact ionization has been studied theoretically and compared to experiments}, 353-390 (2007)}. The dynamics of these ionic clusters has been modeled by means of a trajectory surface hopping method, the Tully's Fewest Switches (TFS) method, in which all the relevant electronic states of the ions and their couplings are taken into account. A very good qualitative agreement is found for all types of clusters, concerning the extensive character of the dissociation and the tendency to form larger fragments when the parent ion size increases. For instance, no trimer fragments are found for clusters smaller than the pentamer. In addition, a very good quantitative agreement is obtained for argon clusters. On the other hand, some discrepancies are found between experiment and theory for krypton clusters: the production of monomers seems underestimated in the simulation. Theoretical results also show that the parent ion dissociation occurs within the first picoseconds, and that most of the dynamics is completed within 10 picoseconds. Despite their success, TFS-like and adiabatic dynamics methods are based on classical mechanics and cannot reach experimental time scales, in the microsecond or millisecond range, whereas large clusters may carry on losing atoms after several nanoseconds. This issue was specifically examined on Ar$_n^+$ clusters (n=20 and 30): a new method that combines a TFS dynamics for the internal conversion, an electronic ground state adiabatic dynamics and phase space theory (PST) was designed and allows to reach the millisecond time scale}, 083401 (2007)}

Entropic effects on the structure of Lennard-Jones clusters

We examine in detail the causes of the structural transitions that occur for those small Lennard-Jones clusters that have a non-icosahedral global minima. Based on the principles learned from these examples we develop a method to construct structural phase diagrams that show in a coarse-grained manner how the equilibrium structure of large clusters depends on both size and temperature. The method can be augmented to account for anharmonicity and quantum effects. Our results illustrate that the vibrational entropy can play a crucial role in determining the equilibrium structure of a cluster.Comment: 13 pages, 9 figure

The Project Russade (Network of Sahelian Universities for Food Security and Environmental Sustainability): the role of universities as engines of development.

The project Russade attempted to give a solution to the lack of skilled professionals in Sahelian countries employed in key structures to ensure food security and improve local productions in a sustainable and environmental respectful perspective.An active network has been established between four universities to improve knowledge and know-how and a new Master course on "Food Security and Environmental Sustainability" was implementedwith an interdisciplinary and multidisciplinary approach.The reinforced capacity of the institutions ensures the training of more competent professionals who can effectively participate in sustainable development emphasizing the links between learning, research and professional practice