Nuclear quadrupole interaction in theFe2+ion including many-body effects

The shielding factor $R$ for the field gradient due to valence electrons in the ${\mathrm{Fe}}^{2+}$ ion has been studied by the many-body perturbation-theoretic procedure including correlation effects. The calculated value of ($1-R$) comes out as 0.93, consistency and correlation effects being, respectively, - 1.3% and - 10.8% of the lowest-order conventional Sternheimer contribution to $R$. Our calculated value of ($1-R$) is about 35% larger than the earlier commonly accepted value of 0.68. This earlier value, which corresponds to an unusually large $R$ as compared to most other systems, is shown to be a consequence of combining a calculated radial contribution to $R$, comparable to our result, with an angular contribution which is a substantial overestimate over both our results and those obtained by Sternheimer by a different procedure. Using our value of ($1-R$), the quadrupole moment $Q$ of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}_{}^m{}_{}{}^{}$, based on Mössbauer data in FeSi${\mathrm{F}}_6$·6${\mathrm{H}}_2$O, is significantly reduced from 0.21 barn obtained using the earlier value of ($1-R$) to a new value $Q=0.15\mathrm{}$ b.