Electronic structure of small iron clusters

We report calculations of energy levels and the charge and spin densities for small free (not embedded) clusters of iron atoms. Our method uses spin-density-functional theory in the local approximation. Single-particle functions are expanded in a basis set of symmetrized linear combinations of Gaussian orbitals. Matrix elements of the electrostatic potential are computed with the aid of a fit to the electron density, again with the use of symmetrized combinations of Gaussian orbitals. The matrix elements of the exchange-correlation potential are evaluated by direct numerical integration using a grid developed for this purpose. The clusters considered are ${\mathrm{Fe}}_7$, ${\mathrm{Fe}}_9$, and ${\mathrm{Fe}}_{15}$. Our results are compared with those obtained by other calculational procedures. The ionization potential of the ${\mathrm{Fe}}_9$ cluster is determined by a transition-state calculation and is compared with experiment.