Calculations using constrained density-functional theory have been carried out for the Ba 1Ϫx K x Bi 1Ϫy Pb y O 3 system, using a full-potential linearized-augmented-plane-wave method and employing fcc supercell geometries with two formula units. The results have been mapped onto Hubbard-type models in order to extract values of interaction parameters U at the Bi sites. Two different mapping procedures have been utilized. The first one is the standard method, based on the comparison of total-energy curvatures. The second method, proposed in the present work, relies on the analysis of single particle energies and yields much smaller numerical errors. For BaBiO 3 interaction parameters are obtained for the following models: ͑i͒ s and p orbitals at Bi and p orbitals at O sites. Here, U s ϭ3.1Ϯ0.4 eV, U sp ϭ1.4Ϯ0.2 eV, U p ϭ2.2Ϯ0.4 eV are found, ͑ii͒ s͑Bi͒ and p͑O͒ orbitals, yielding U s ϭ1.9Ϯ0.7 eV, and ͑iii͒ an effective one-band model, leading to U s ϭ0.6Ϯ0.4 eV. Further studies have been performed for breathing distorted BaBiO 3 and for various Ba 1Ϫx K x Bi 1Ϫy Pb y O 3 alloys using virtual crystal approximations. The resulting U values are somewhat larger than for pure BaBiO 3 . Thus, in all cases, the values of Bi U parameters are found to be positive. There is no indication of a negative U of electronic origin.

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We present ab initio full-potential linearized augmented plane wave (LAPW) band structure calculations for BaBiO3. We focus on total energies and electric field gradients at the oxygen site and present results for the breathing mode in the experimentally observed cubic and monoclinic structures of doped and undoped Ba1-xKxBiO3.

,CuO, the longitudinal, frequency and wavenumber dependent dielectric function is calculated on the basis of a two dimensional LCAO model. The results are in agreement with much more elaborated investigations, provided these are available.Es wird fur La,Cu04 die longitudinale, frequenz-und wellenzahlabhangige Dielektrizitatsfunktion auf der Grundlage eines zweidimensionalen LCAO Modells berechnet. Die Ergebnisse sind in obereinstimmung mit erheblich aufwendigeren Rechnungen, soweit diese verfiigbar sind.