Cluster-forming systems of ultrasoft repulsive particles: statics and dynamics

6 pages.-- PACS nrs.: 64.70.Dv; 82.30.Nr; 61.20.Ja; 82.70.Dd.-- Special issue of Computer Physics Communications based on the Conference on Computational Physics 2007 - CCP 2007.In this paper, we present a short review as well as novel results on a recently established counterintuitive phenomenon of cluster aggregation of particles that interact via purely repulsive interactions. We demonstrate how repulsion can lead to clustering provided that the interaction allows full particle overlaps and also displays negative Fourier components. The formation of crystals with average site occupancy that scales linearly with density, and which is in general a noninteger number, is demonstrated by means of density functional theory and Monte Carlo simulations. Noninteger average occupancy of lattice sites is sustained by incessant hopping processes in the crystal, which we observe and quantitatively analyze by means of Molecular Dynamics simulations. The simultaneous presence of relaxative dynamics on the single-particle and arrested dynamics on the collective level leads to an unusual interplay between the two and to novel scenarios of dynamical arrest in the cluster crystal.C.N.L. acknowledges partial support from the DFG through the SFB-TR6, Project Section C3. B.M.M., D.G., and G.K. acknowledge financial support by the Österreichische Forschungsfond (FWF) under Project Nos. P17823-N08 and P19890-N16, and the Hochschuljubiläumsstiftung der Stadt Wien under Project No. 1080/2002. Further, B.M.M. thanks the Österreichische Forschungsgemeinschaft for financial support. This work was partly carried out under the HPC-EUROPA Project (RII3-CT-2003-506079), with the support of the European Community—Research Infrastructure Action under the FP6 "Structuring the European Research Area" Programme. A.J.M. acknowledges financial support from SoftComp (NMP3-CT-2004-502235) and DIPC-Spain.Peer reviewe