Semiempirical method for transition metal complexes with open -shells

Semiempirical method for transition metal complexes with open d-shells 

Transition metal complexes (TMCs) represent another, somewhat better known, Holy Grail of the semiempirical theory. The HFR-based semiempirical methods and the DFT-based methods suffer from structure deficiency, which does not allow it to reproduce relative energies of electronic states of different spin multiplicity within their respective frameworks without serious ad hoc assumptions. 

The difficulties arise precisely when modeling is to be applied to molecules involving transition metal atoms, mainly of the second half of the first transition row. Even among the TMCs formed by these atoms, the problems seem not to be uniformly 

Further analysis is based on the idea that the characteristic experimental behavior of different classes of compounds and the suitability of those or other models used to describe this behavior is ultimately related to the extent to which chromophores responsible for the observed behavior and physically present in the molecular system are reflected in these models by adequate electron groups. The TMCs of interest, can be physically characterized as those bearing the 4-shell chromophores. (The analogy between the chromophore concept and McWeeny s theory for the special case of TMCs has also been noticed early in a remarkable work [149]). The basic features in the electronic structure of TMCs of interest, distinguishing these compounds from others, are the following  

These molecules contain strongly correlated electrons in the partially filled valence 4-shell of the transition metal central atom. 

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