Stability of the graphite and diamond phases of finite carbon cluster

The stability of particulate carbon formed in the detonation of high explosives has been investigated with first principles and semiempirical molecular orbital calculations carried out on carbon clusters. The dangling surface bonds were capped wit/r hydrogen atoms and the surface contributions to the cohesive energy were removed by extrapolation as a function of the cluster size. Comparison of the calculated heat of formation of graphite and diamond particles as a function of size predicts that the graphite phase becomes more stable for IO⁴ -10<sup5</sup> carbon atoms. Calculations were also carried out on geometry optimized carbon clusters without capping atoms, resulting in reconstructed cluster surfaces that may be a more realislic model for particulate carbon formed under the extreme conditions of detonation. The calculated energy barrier for tbe conversion of a graphitic cluster to the cubic diamond structure was in good agreement with calculations on