The annealing of a small nanodiamond cluster at 1500 K is studied by molecular dynamics. The transformation of the particle in an almost fully graphitized carbon onion is observed. The remaining 17% of sp(3) atoms are delocalized on the whole particle, both under the form of isolated point defects and of small diamond-like clusters separating large graphite-like domains. It is also shown, that the Berendsen thermostat, previously used to fix temperature in such simulations, transfers kinetic energy from internal to global motions of the cluster. This can lead to severe artifacts like the freezing of the graphitization process. (c) 2008 Elsevier B.V. All rights reserved.