Wannier-like functions and tight-binding parametrization for the manganese bands in CaMnO3
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We study the electronic band structure of CaMnO3, in order to understand the origin of the dispersion of the Mn(eg) bands, which is in contrast with the predicted dispersionless bands within the Anderson-Hasegawa double-exchange model with infinite Hund's-rule coupling. A downfolding technique within the newly developed muffin-tin orbital-based method is used to analyse the density-functional band structure obtained in the local spin density approximation. The finite Hund's coupling parameter in realistic situations allows the same-spin bands on the two manganese sublattices to mix producing a large dispersion. The calculated Wannier functions for the Mn(eg) bands also show large oxygen character at sites further away from nearest oxygen sites causing long-ranged Mn-Mn hopping processes.