Use of the forced populating of d orbitals to eliminate oscillations during the determination of a self-consistent charge approximation

The extended Hueckel (EH) method with the determination of a self-consistent approximation with respect to the charges of the atoms (the charge iterative extended Hueckel or IEH method) has been used to study the electronic structure of complexes of metals with porphyrins (P) and to explain the observed spectroscopic laws. The authors devised a modified variant of the IEH program with the forced populating of the d orbitals of the metal by electrons during the determination of the self-consistent approximation. The experimental data from gamma resonance spectroscopy and ESR for many FeP complexes allow them to draw a conclusion regarding the spin state in which the metal ion exists [for Fe(II) it may be S = 0, 1, or 2; for Fe(III) it may be S = 1/2, 3/2, or 5/2]. Therefore, the following method can be used. In each step of the iteration process, the composition of the MO's near the frontier is analyzed, and the MO's of the assigned symmetry corresponding to the 3d orbitals of the metal are singled out. Fixed values of occupancy values (fractional values are also possible), which guarantee the assigned spin state of the metal are assigned to these MO's. Different populating schemes (for the assigned spin S) naturally give different results. The important fact is that in all cases the method of forced occupancies eliminates the oscillations of the charges in the iterations