Density-functional calculations for small iron clusters: ${\mathrm{Fe}}_{\mathit{n}}$, ${\mathrm{Fe}}_{\mathit{n}}^{+}$, and ${\mathrm{Fe}}_{\mathit{n}}^{\mathrm{-}}$ for n≤5

The optimum geometries, electronic and magnetic structures, bond dissociation energies (BDE’s), binding energies (${\mathit{D}}_{\mathit{e}}$’s), ionization potentials (IP’s), and electron affinities (EA’s) of small iron clusters are studied by means of a .ul2 linear combination of Gaussian-type orbitals–local and/or nonlocal spin-density method. At the nonlocal level and with respect to nonspherical iron atoms the calculated ${\mathit{D}}_{\mathit{e}}$’s are 1.04, 1.41, 1.87, and 2.20 eV/atom for ${\mathrm{Fe}}_2$, ${\mathrm{Fe}}_3$, ${\mathrm{Fe}}_4$, and ${\mathrm{Fe}}_5$, respectively. The calculated IP’s are 8.16, 7.01, 6.34, 6.20, and 6.52 eV for Fe, ${\mathrm{Fe}}_2$, ${\mathrm{Fe}}_3$, ${\mathrm{Fe}}_4$, and ${\mathrm{Fe}}_5$, respectively, in reasonable agreement with their experimental counterparts of 7.9, 6.3, 6.4–6.5, 6.3–6.5, and 5.9–6.0 eV, and also are close to those obtained by means of ab initio techniques. The lowest-energy states are those with a maximum number of nearest-neighbor (short) bonds, and with high magnetic moments (3, 2.67, and 3 spins per atom for ${\mathrm{Fe}}_2$, ${\mathrm{Fe}}_3$, and ${\mathrm{Fe}}_4$; in ${\mathrm{Fe}}_5$ the magnetization is unevenly distributed and ranges from 2.90 to 3.31 spins per atom) coupled ferromagnetically. The gain in magnetic energy promotes the close-packed structures (for n=3 and 4) or small distortions into somewhat more open geometries (for n=5).