According to ligand field theory, which itself evolved out of crystal field theory, the presence of the ligands within the (perfect) octahedral crystal environment present in the cubic variant of Prussian blue based materials lifts the degeneracy of the d-orbitals. The d xy , d xz and d yz orbitals, collectively called t 2g states after Mulliken, are lowered in energy with respect to the barycenter, while the d x 2 and d z 2 -y 2 orbitals (or e g states) increase their energy. The amount of energy the orbitals are separated by is called the crystal field splitting energy (CFSE), which is dependent on the ligand environment around the TM. This energy is distributed in such a way that the energy of the barycenter still exactly coincides with the energy of the degenerate d-orbitals. While empty or fully filled shells of electrons do not gain energy via this splitting, it can be energetically beneficial for partially filled shells.