Structural dependence of the cohesive-energy, abundancy and bond-length of transition-metal clusters

We calculate the size-dependence of the cohesive-energy, abundancy and average bond-length of small Fe n- and Ni n-clusters. A tight-binding Hubbard Hamiltonian in the Hartree-Fock approximation together with the second moment approximation is used. Results for different assumed cluster structures are compared and discussed. In particular we obtain that bcc- like clusters with n = 6 and 15, and fcc-like clusters with n = 5, 7, 13 and 19 are particularly stable, in agreement with experimental observations for Fe n and Ni n, respectively. Allowing uniform relaxation for each assumed structure we obtain for small clusters a reduction of the equilibrium bond-length of about 10% with respect to bulk. The role of magnetism for the stability of the clusters and limitations of the present calculations are discussed.