WHAT CAN CALCULATIONS EMPLOYING EMPIRICAL POTENTIALS TEACH US ABOUT BARE TRANSITION-METAL CLUSTERS

The implications for transition-metal clusters of theoretical results for systems containing 10-148 atoms bound by empirical potentials have been considered. The effects of the range of the interatomic pair pot ential and anisotropy on the potential-energy surface are now quite we ll understood. For example, as the range decreases the favoured morpho logy changes from icosahedral to decahedral and then to cuboctahedral. Since strain increases with size the crossover between electronic and geometrical 'magic numbers' exhibited by alkali-metal clusters can be rationalised. Calculations employing specific potentials designed to represent face-centred-cubic transition metals enable the study of cha nges in morphology and surface migrations in clusters of these element s. Single-step mechanisms exist for highly co-operative rearrangements between different structures, but the associated barriers scale as th e total number of atoms. Hence, at larger size the same mechanisms are mediated by a series of transition states. The barriers for surface p rocesses are comparable to those deduced experimentally and theoretica lly for bulk surfaces. It is predicted that icosahedral order is 'froz en in' at relatively small size and Mackay icosahedra grow via anti-Ma ckay and then Mackay overlayers.