The electronic structure of small transition metal clusters

In this paper we report the energy level distributions for transition metal clusters obtained by symmetry-broken and symmetry-constrained SCF methods. Investigated clusters were Fe4, Ni4, Cu4, Zn4, Fe6, Ni6, Cu6, and Zn6. Contracted Gaussian-type basis sets of the same size generated by the method of Tatewaki and Huzinaga were used throughout the paper. It will be shown that the d-hole localization model as well as the model with a two-electron-like ionization process is essential to discuss the electronic structure of the positive ion states, and the models bring the energy level distribution of the clusters rather close to the density of the state of the corresponding metals. The calculated first ionization potentials agree quite well with those of experiment. For example, the experimental ionization potential for Fe4 is between 6.3 and 6.5 eV, while calculated values are 5.9 and 6.3 eV for the 3d electron ionization (d−1) and 4s electron ionization (s−1), respectively. If we define the Fermi level of the clusters as the first ionization potential, the Fermi level for Fe4 is composed of the d−1or d−1 + s−1 state. The Fermi level for Ni4 is found to be s−1or s−1 + d−1. Those of Cu4 and Zn4 are definitely of the s−1 state. Further discussion is also presented for larger clusters of M6 (M = Fe, Ni, Cu, and Zn). Keywords: transition metal clusters, ionization potential, abinitio SCF calculations, energy level distributions.