Midinfrared spectrum of undoped cuprates: d-d transitions studied by ab initio methods

We present results of ab initio calculations for d-d transitions, which ari se in the midinfrared spectrum of undoped cuprate compounds. It has been su ggested that these transitions arise at energies as low as 0.4 eV in La2CuO 4 and Sr2CuO2Cl2. We study the differences in d-d transition energies in a series of cuprates that contains compounds in which the Cu ions are sixfold , fivefold, or fourfold coordinated. Furthermore, we analyze the dependence of the 3d(x2-y2)-->3d(z2) excitation energy on the ratio of the in-plane a nd apex copper-ligand distances in the model system CuO. Our cluster calcul ations do not support the assignment of the 0.4-1-eV band to phonon and mag non sidebands of a d-d transition. On the other hand, we confirm the interp retation of the peak around 1.7 eV observed in CuGeO3 as arising from d-d t ransitions.