Limitation of semi-empirical mo-calculations in deriving charge densities ρ(0) in iron-oxygen compounds

Abstract

For fifteen iron-oxygen compounds we carry out semi-empirical MO cluster calculations. The derived electronic structure is used to calculate electron charge densities ρ(0) and electric field gradients V pq at the iron nucleus. The ρ(0)-values correlate with experimental isomer shifts δ, however, extreme cases like the Fe(VI)-compound BaFeO 4 are beyond the scope of accuracy of our method. We discuss variations of our calculational method, with one of them leading to ρ(0)-values for all fifteen compounds (including BaFeO 4) which satisfy the relation Δδ = αΔρ(0). The isomer shift calibration constant is in the range -0.195 mms −1 a 0 3 to -0.25 mms −1 a 0 3. Calculated quadrupole splittings are comparable with experimental data.