Chalcogenide perovskites are proposed for photovoltaic applications. The predicted band gaps of CaTiS3, BaZrS3, CaZrSe3, and CaHfSe3 with the distorted perovskite structure are within the optimal range for making single-junction solar cells. The predicted optical absorption properties of these materials are superior compared with other high-efficiency solar-cell materials. Possible replacement of the alkaline-earth cations by molecular cations, e.g., (NH3NH3)(2+), as in the organic-inorganic halide perovskites (e.g., CH3NH3PbI3), are also proposed and found to be stable. The chalcogenide perovskites provide promising candidates for addressing the challenging issues regarding halide perovskites such as instability in the presence of moisture and containing the toxic element Pb.

Cation-vacancy induced intrinsic magnetism in GaN and BN is investigated by employing density-functional theory based electronic structure methods. The strong localization of defect states favors spontaneous spin polarization and local moment formation. A neutral cation vacancy in GaN or BN leads to the formation of a net moment of 3 muB with a spin-polarization energy of about 0.5 eV at the low density limit. The extended tails of defect wave functions, on the other hand, mediate surprisingly long-range magnetic interactions between the defect-induced moments. This duality of defect states suggests the existence of defect-induced or mediated collective magnetism in these otherwise nonmagnetic sp systems.

Contrary to previous reports, we show that the conventional GW (the so-called G⁰W⁰) approximation can be used to calculate accurately the experimental band gap (∼3.6  eV) of ZnO. The widely discussed underestimate of the quasiparticle gap of ZnO within the GW method is a result of an inadequate treatment of the semicore electrons and the slow and nonuniform convergence in the calculation of the Coulomb-hole self-energy in previous studies. In addition, an assumed small kinetic energy cutoff for the dielectric matrix may result in a false convergence behavior for the quasiparticle self-energy.

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