Crystal field stabilization energy (CFSE)

The crystal field stabilization energy is defined to be the energy difference between the actual configuration of the negatively charged ligands and the energy of a negatively charged sphere surrounding the metal cation. The sphere is chosen so that the locations of the negatively charged ligands are points of the sphere. The negatively charged sphere is called a barycenter In the barycenter, the $d$ orbitals are perturbed so that all of the $d$ orbital energies are raised, but they are all perturbed in the same way, so they are fivefold degenerate. For an octahedral arrangement, we find that the $t_{2g}$ orbitals are $(2/5)\Delta_0$ below the energy level of the barycenter while the $e_g$ orbitals are $(3/5)\Delta_0$ above the level of the barycenter.

Referring to the diagram above for the Mn$^{3+}$, the low-spin configuration has a total CFSE is $-(8/5)\Delta_0$, while in the high-spin configuration, the CFSE is $-(3/5)\Delta_0$.