Spin state transition and covalent bonding in LaCoO3

We use the dynamical mean-field theory to study a p-d Hubbard Hamiltonian for LaCoO3 derived from ab initio calculations in local density approximation (LDA + DMFT scheme). We address the origin of local moments observed above 100 K and discuss their attribution to a particular atomic multiplet in the presence of covalent Co-O bonding. We show that in solids such attribution, based on the single ion picture, is in general not possible. We explain when and how the single ion picture can be generalized to provide a useful approximation in solids. Our results demonstrate that the apparent magnitude of the local moment is not necessarily indicative of the underlying atomic multiplet. We conclude that the local moment behavior in LaCoO3 arises from the high-spin state of Co and explain the precise meaning of this statement.