CECAM - Simulations of hard bodies

It is currently possible in an efficient way to calculate the distance between two hard convex objects with a generic shape and to predict their collision [ED4HB]. The first application of such algorithms has been tested on the molecular dynamics of a systems of classical hard ellipsoids of revolution [DeMicheleHE06]. The same methods allows to simulate shaped step interactions[DeMichelSticky06]. The recent introduction of an algorithm for the rotation of objects with a generic inertia tensor completes such approach [vanZonCondMat]. Moreover,following the lines of [BD4HS], it seems to be possible to extend classical Molecular Dynamics simulation to the Brownian Dynamics simulation of interest in the field of colloids and suspensions. A renewed interest for assemblies of hard shaped objects stems from the field of colloidal science; nevertheless,simulations of the dynamics of hard objects [Allen89,Allen93] needs to be revived and theoretical investigations on the dynamics of shaped objects reconsidered. Besides, we believe that there is a potential interest in several cross-disciplinary scientific and technological fields. Such belief stems from an analysis of the current investigations in several fields: * theory of molecular liquids: while hard-spheres are widely understood and form the reference system for many theories of the dynamics and the statics, there no such understanding for shaped objects [GrayGubbinsBook] * granular fluids: simulations of shaped particles are practically limited to hard needles * geo-mechanics: soil rheology employs mostly models and simulations where grains are spherical or, in the most sophisticated cases, ellipsoidal [Ouadfel99,Ouadfel01]. The new rheology introduced by switching from spheres to elongated objects should induce to take account of shape as a must, not as an optional. * powders: same situation than in geo-mechanics [Ouadfel99,Antony04]. * fruit & vegetables: damages due to processing, transportation and handling are studied via simulations of spheres that represent potatoes, tomatoes, apples [LoodtsMSTh]. Damages are predicted looking at the collision points on the spheres; from our experience on ellipsoids, we believe that at least the collision frequency and the magnitude of the impact is deeply influenced even by a little elongation of the objects. Moreover, experiments for the mechanical characterizations require at least a description in terms of an ellipsoidal object [Cherng05]. * colloids and nanoassembly: despite the possibility of controlling the shape of the particles [vanDillen2004], hard spheres remain the main reference system. * lyotropic liquid crystals: several overlap criteria [Allen93,Het99,Blaak99] allow the simulation of specific hard objects and point out the importance of the shape on the appearance of new liquid-crystal phases. No general method to investigate and quantify such effect exists for at least convex objects. * coarse-grained proteins: from experimental data, at least Human Serum Albumin can be modeled as an hard ellipsoid with a repulsive stepwise potential [Sjoberg97]. * go-models [Go83] for folding: rigid sub-sequences of a protein could be modelled by a single shaped unit speeding up the simulations. * computer science, computer graphics, game design, robotics, virtual reality: distance calculations and collision prediction are the core for the realization of virtual environments and robotic manipulations; again, most algorithm are specific and not shape-independent [Ju01,Eberly01]. In conclusion, we believe that it is timely to promote a cross-disciplinary meeting on the subject in order to create an enlarged community sharing the same interests on simulating hard bodies

Simulations of hard bodies

CECAM 46 allée d'Italie 69007 Lyon France

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