Multiplet ligand-field theory using Wannier orbitals

We demonstrate how ab initio cluster calculations including the full Coulomb vertex can be done in the basis of the localized Wannier orbitals which describe the low-energy density functional (local-density approximation) band structure of an infinite crystal, e.g., the transition-metal 3d and oxygen 2p orbitals. The spatial extent of our 3d Wannier orbitals (orthonormalized Nth-order muffin-tin orbitals) is close to that found for atomic Hartree-Fock orbitals. We define ligand orbitals as those linear combinations of the O 2p Wannier orbitals which couple to the 3d orbitals for the chosen cluster. The use of ligand orbitals allows for a minimal Hilbert space in multiplet ligand-field theory calculations, thus reducing the computational costs substantially. The result is a fast and simple ab initio theory, which can provide useful information about local properties of correlated insulators. We compare results for NiO, MnO, and SrTiO3 with x-ray absorption, inelastic x-ray scattering, and photoemission experiments. The multiplet ligand-field theory parameters found by our ab initio method agree within similar to 10% with known experimental values.