Ab initio study of antiferromagnetic rutile-type FeF₂

The periodic unrestricted Hartree-Fock method has been used to calculate the ground-state spin-polarized wave function and total energy of tetragonal FeF₂. Contracted Gaussian-type functions represent the atomic orbitals of Fe and F atoms. The nature of the antiferromagnetic insulator, with a d-d type band gap, is reproduced correctly by the calculations. On the basis of the density of electron states, Mulliken population data, and charge- and spin-density maps, the d_xy²/d_xz¹d_yz¹ splitting of t_2g⁴ states is proved for the Fe²⁺ ion, consistent with the Jahn-Teller distortion of the FeF₆ octahedron. A limited charge back transfer from F^- to Fe²⁺ is observed, with z_Fe=+1.84‖e‖. The equilibrium structural configuration and its evolution with pressure up to 12 GPa have been calculated, including internal relaxation. The athermal equation of state, structural, and elastic parameters show a good agreement with experimental data.