Ground-state properties of small iron clusters

Ground-state structures of small iron clusters, N≤18, are determined from simulated-annealing calculations. Total energies are calculated using the effective-medium theory, and the d-band energy is included using the tight-binding approximation. The main effects of the partially filled d band are a higher symmetry of the ground-state structures than those assumed by simple metals such as Na and Cu and a tendency to produce hollow structures. We find a correlation between the presence of unscreened atoms in the iron-cluster ground states and measured ionization potentials and reactivities with ${\mathrm{H}}_2$.