Electron correlation effects in small iron clusters

We present results of first-principles calculations of structural, magnetic, and electronic properties of small Fe clusters. It is shown that, while the lowest-energy isomers of Fe3 and Fe4 obtained in the framework of density functional theory within the generalized gradient approximation (GGA) are characterized by Jahn-Teller-like distortions away from the most regular shapes (which is in agreement with other works), these distortions are reduced when electron correlation effects are considered explicitly as within the GGA+U approach. At the same time, the magnetic moments of the clusters are enhanced with respect to the pure GGA case, resulting in maximal moments (in the sense of Hund's rules) of 4 μB per atom for the ground state structures of Fe3 and Fe4, and a total moment of 18 μB for Fe5. This already happens for moderate values of the Coulomb repulsion parameter U ∼ 2.0 eV and is explained by changes in the electronic structures of the clusters.