Spin-state transition and spin-polaron physics in cobalt oxide   perovskites: ab initio approach based on quantum chemical methods
  A fully ab initio scheme based on quantum chemical wavefunction methods is used to investigate the correlated multiorbital electronic structure of a 3d-metal compound, LaCoO3. The strong short-range electron correlations, involving both Co and O orbitals, are treated by multireference techniques. The use of effective parameters like the Hubbard U and interorbital U', J terms and the problems associated with their explicit calculation are avoided with this approach. We provide new insight into the spin-state transition at about 90 K and the nature of charge carriers in the doped material. Our results indicate the formation of a t4e2 high-spin state in LaCoO3 for T>90 K. Additionally, we explain the paramagnetic phase in the low-temperature lightly doped compound through the formation of Zhang-Rice-like O hole states and ferromagnetic clusters. 
L. Hozoi, U. Birkenheuer, H. Stoll, and P. Fulde