The band structure of small iron clusters from Fe1 to Fe6

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Abstract
The band structures of Fe clusters Fe1–Fe6 were investigated by symmetry‐adapted and symmetry‐broken SCF calculations. For Fe2, configuration interaction (CI) calculations were performed at the equilibrium internuclear distance (Re) of 2.02 Å and at another long distance of 2.60 Å. The ground state of Fe2 at Re was hardly described by a single configuration, which was also the case for the ionized states. The Fermi level (the first ionized state) of Fe2 at Re was composed of a 3d electron ionized state (3d−1), and it lay 4.8 eV below the vacuum level by the CI calculations. The result was 1 eV small compared with the experimental value given by Rohlfing and co‐workers. On the other hand, the single configuration approximation (SCF) worked for the larger bond distance (2.60 Å). The Fermi level again consisted of the 3d−1 state, but the d hole was localized at one of the constituent atoms. This was also observed in larger clusters. In the SCF approximation, the atomic 3d ionization potential was considerably smaller (2.4 eV) than the experimental value. Therefore, atomic correlation correction was necessary for Fe2–Fe6, and the calculated Fermi levels modified with the atomic correlation correction were 5.3, 5.9, 5.9, 6.4, and 5.9 eV for Fe2, Fe3, Fe4, Fe5, and Fe6, respectively, which were reasonably close to the experimental values of 6.3, 6.4–6.5, 6.3–6.5, 5.9–6.0, and 5.8–6.0 eV. The ionized states with two d holes were found near the Fermi level for larger clusters.