DENSITY-FUNCTIONAL CALCULATIONS FOR SMALL IRON CLUSTERS - FEN, FEN+, AND FEN- FOR N-LESS-THAN-OR-EQUAL-TO-5

The optimum geometries, electronic and magnetic structures, bond dissociation energies (BDE's), binding energies (D(e)'s), ionization potentials (IP's), and electron affinities (EA's) of small iron clusters are studied by means of a 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 D(e)'s are 1.04, 1.41, 1.87, and 2.20 eV/atom for Fe2, Fe3, Fe4, and Fe5, respectively. The calculated IP's are 8.16, 7.01, 6.34, 6.20, and 6.52 eV for Fe, Fe2, Fe3, Fe4, and Fe5, 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 Fe2, Fe3, and Fe4; in Fe, 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). The importance of the s electrons for the bonding properties increases with increasing cluster size; starting from predominantly localized d bonds on earlier clusters, the chemical bond evolves to show a delocalized s pattern at the Fermi level for the larger clusters. The equilibrium bond lengths, for the computed ground states (2.00, 2.10, and 2.22 angstrom for Fe2, Fe3, and Fe4, respectively; whereas in Fe5 they range from 2.23 to 2.63 angstrom) are much shorter than the shortest distance in the bulk, 2.48 angstrom, although for the pentamer some bond lengths approach the bulk value. The fully optimized cations, Fe(n)+, and anions Fe(n)-, occur in similar structures as those of the ground states; in general, Fe(n)+ show a larger structural relaxation than Fe(n)-.