Ligand to Metal Donation: In oxides such as PuO 2 or UO 2 or CeO 2 , the oxo-dianions are stabilized by the electrostatic field in the crystal as well as by covalent interactions with the metal ions. Ligand-to-metal (LM) donation via dative bonding (donation) occurs to varying degrees for essentially all metal ions coordinated by ligands, along with metal-to-ligand back-donation, depending on the ligand type and the metal ion's electron configuration. LM donation is a manifestation of electron delocalization, namely, a ligand electron or-perhaps more typically-an electron pair delocalizing toward the metal. In the commonly applied orbital-based framework of QM, such as DFT or Hartree-Fock (HF) theory, the delocalization representing LM donation manifests itself in the form of in-phase (bonding) linear combinations of occupied ligand-centered and vacant metal-centered fragment orbitals or atomic orbitals (AOs). In MC-WFT calculations, LM donation may either be described via bonding orbitals or alternatively by the mixing of a charge-transfer configuration into the ground-state wavefunction if a basis of localized metal-and ligand-centered orbitals is used. Irrespective of the description of LM donation, either via in-phase mixing of fragment orbitals or via mixing of charge-transfer configurations into the wavefunction, the result is an increase of the electron density in the metal's valence shells. Consequently, in the presence of LM donation the electron count at the metal, suitably defined, is higher than the formal electron count of the ion, i.e., n f > n open , and the metal ion's charge is less positive than its formal charge.