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 arise in the midinfrared spectrum of undoped cuprate compounds. It has been suggested that these transitions arise at energies as low as 0.4 eV in La2CuO4 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 and apex copper-ligand distances in the model system CuO. Our cluster calculations do not support the assignment of the 0.4-1-eV band to phonon and magnon sidebands of a d-d transition. On the other hand, we confirm the interpretation of the peak around 1.7 eV observed in CuGeO3 as arising from d-d transitions.