Electronic structure, pressure- and temperature-dependent charge densities, and electric field gradients in FeF2

We derive the electronic structure of Fe${\mathrm{F}}_2$ from molecular-orbital (MO) cluster calculations. Fe${\mathrm{F}}_2$ is represented by a Fe${\mathrm{F}}_6^{4-}$ cluster. Pressure- and temperature-dependent cluster geometries are taken from the literature. The five configurations ${}_{}{}^5{}_{}{}^{}B_{1g}^{}{}_{}{}^{}$, ${}_{}{}^5{}_{}{}^{}B_{2g}^{}{}_{}{}^{}$, ${}_{}{}^5{}_{}{}^{}B_{3g}^{}{}_{}{}^{}$, ${}_{}{}^5{}_{}{}^{}A_g^{}{}_{}{}^{}$, and ${}_{}{}^5{}_{}{}^{}A_{}^{\prime }{}_g{}^{}$, which we use to take into account configuration-interaction and spin-orbit coupling, are based on one-electron-MO functions. The energy separations of these configurations are scaled to match a particular experimental $\Delta E_Q$ value; reasonable agreement is thereby obtained over a range of temperature and pressure. The calculated pressure- and temperature-dependent electron charge densities and electric-field-gradient tensors at the iron nucleus are consistent with experimental isomer shifts, quadrupole splittings and asymmetry parameters in the paramagnetic phase as well as in the antiferromagnetic phase. Obtained energy separations are comparable with optical data.