Hole localization in ionized and bound excitation states of monovalent copper halides

Abstract

The results of self-consistent field calculations on the ground states, ionized stated and some excited states of the clusters [CuCl₄]³⁻, [CuBr₄]³⁻, [Cu₄Cl]³⁺ and [Cu₂Cl]⁺ are presented and discussed. The Madelung field of the rest of the crystal has been incorporated in the calculations. High and low point-group symmetries are considered, showing the importance of relaxing symmetry constraints. The calculated ionization energies show good overall agreement with experiment; however, the calculated crystal field splitting of the 3d levels is much smaller than the observed one. The calculated 3d–4s-like excitation energies are very sensitive to both the external field and the basis sets used, and more studies are required before firm conclusions can be drawn. The results indicate that at the level of the single-configuration SCF approximation, open-shell orbitals describing 3d holes are strongly localized on copper. Their ‘covalency’ is small, so that a seemingly necessary contribution to an understanding of the spectral properties is missing at this level of approximation, which includes one-electron band theory.