Structure and bonding in small aluminum clusters

Density-functional calculations with simulated annealing have been performed for aluminum clusters ${\mathrm{Al}}_{\mathit{n}}$ up to n=10. There are many local minima in the energy surfaces, with a rich variety of structures and spin multiplicities. With increasing cluster size we find transitions from planar to nonplanar structures at n=5, and to states with minimum spin degeneracy at n=6. There are stable isomers of ${\mathrm{Al}}_{5–}$${\mathrm{Al}}_{10}$ with buckled planar structures reminiscent of the layers in crystalline α-gallium. All structures show regular patterns of bond and dihedral angles. Trends in binding and ionization energies are compared with experiment and with the predictions of other calculations.