Ligand field theory energy level diagram

IV. Molecular Orbital Energy Level Diagrams and The Third Revolution in Ligand Field Theory... 

Thus in order to calculate an energy-level diagram and/or details of magnetic behavior in ligand field theory, one proceeds in the same manner as in crystal field theory except that, instead of assuming the free-ion values for A, B and C, one either assumes somewhat smaller ones or leaves them as parameters to be evaluated from the experimental observations. In this way all the computational and conceptual advantages of the simple electrostatic theory are preserved while allowance is made—in an indirect and admittedly artificial way—for the consequences of finite orbital overlap. One also bears in mind that there are other consequences of the overlap, for example, electron delocalization. 

At first sight, the main interest of monocoordinated complexes for Quantum Chemistry lies in the simplicity of their energy-level diagrams, as given by the standard ligand field or molecular orbital theories [11]. It is then easy to identify various electronic energy components ( i.e., electrostatic, polarisation, exchange contributions and so on. ..) in the theoretical results [12,13], at least at the SCF level. 

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