Effect of many-body interactions on isomer shift in iron compounds

Using many-body perturbation theory, the correlation contributions to the total electronic densities at the nuclei of Fe⁺³, Fe⁺², and Fe⁰ systems have been evaluated. It is shown that while individual correlation diagrams might contribute as much as (1-2)a₀^-3 to the density at the nucleus, the net correlation corrections to the difference of densities at Fe⁺² and Fe⁺³ ions and at Fe⁰ and Fe⁺² ions are quite small, namely 0.03a₀^-3 and 0.3a₀^-3, respectively, due to substantial cancellation among individual diagrams. The latter number is found to be larger than the former because of the correlation effect associated with $4s$ electrons. With the many-body contribution known, we have analyzed the importance and present accuracy of other factors involved in the determination of the isomer-shift constant from electron-density calculations.