FeAs$_2$ formation and electronic nematic ordering: Analysis in terms of   structural transformations
  By combining DFT-based computational analysis and symmetry constraints in terms of group-subgroup relations, we analysed the formation of the native crystalline structure of loellingite FeAs$_2$. We showed that the ground state of the material exhibits the ordered patterns of the electronic localization which are mainly associated with iron $3d_{x^2 - y^2}$ orbitals and can be characterized in terms of nematic-like ordering. The ordering is the result of the close interplay of the lattice and the electron degrees of freedom. In a structural aspect, it pursues an energy quest to select the orthorhombic crystal lattice attributed to the Pnnm space group. In a charge aspect, the ordering is connected with the valence charge density redistribution that not only provides a high electronic polarizability but also gives rise to an extra-large magnitude of the negative component of the dynamical p-d charge transfer. 
A. Pishtshev and P. Rubin
cond-mat.mtrl-sci cond-mat.str-el