Comparison of ab initio and density functional calculations of electric field gradients: The Fe-57 nuclear quadrupole moment from Mossbauer data

The difficulty in accurate determination of the nuclear quadrupole moment o f the first I=3/2 excited nuclear state of Fe-57 from electronic structure calculations of the iron electric field gradient combined with Mossbauer me asurements of the nuclear quadrupole splitting in the isomer shift is addre ssed by comparing ab initio with density functional calculations for iron p entacarbonyl, Fe(CO)(5), ferrocene, Fe(C5H5)(2), and the (5)Delta (g) elect ronic ground states of FeCl2 and FeBr2. While the ligand field gradient ten sor components change relatively little with the method applied, the iron e lectric field gradient is sensitive to the specific density functional used . Single reference many-body perturbation theory for electron correlation a lso performs poorly for the iron electric field gradient and shows extreme oscillatory behavior with a change in the order of the perturbation series. Even with larger basis sets and coupled cluster techniques a precise value for the iron electric field gradient could not be determined from electron ic structure calculations due to limitations in the theoretical procedures. In order to avoid uncertainties in the measured isomer shift which enters into the nuclear quadrupole coupling constant we determined the Mossbauer s pectrum of Fe(C5H5)(2) between temperatures of 4.2 and 295 K. In this range two phase transitions are observed, but the quadrupole splitting is not ve ry dependent on the solid state structure in each phase. Solid state effect s for the Fe(CO)(5) are determined by comparing the iron electric field gra dient of the isolated molecule with the value obtained from first principle solid state calculations at various levels of theory. These calculations s how that the influence of near neighboring effects to the iron electric fie ld gradient is small. Fully relativistic Dirac-Hartree-Fock calculations fo r Fe(CO)(5) reveal that relativistic effects for the iron electric field gr adient are small as well. Fe(CO)(5) is therefore an ideal test molecule for the determination of an accurate nuclear quadrupole moment from electronic structure calculations if combined with an experimental nuclear quadrupole coupling constant. Our best estimate for the Fe-57 nuclear quadropole mome nt is 0.14(2) barn in reasonable agreement with recent nuclear structure ca lculations. (C) 2001 American Institute of Physics.