Energetic stability and magnetic properties of MnO in the rocksalt, wurtzite, and zinc-blende structures: Influence of exchange and correlation

We investigate the relative energetic stability and the magnetic properties of MnO in the rocksalt (rs), wurtzite (wz), and zinc-blende (zb) structures using density-functional theory with different approaches to exchange and correlation: the semilocal generalized-gradient approximation (GGA), the GGA+U method with an additional on-site interaction U, and the spatially nonlocal hybrid functional HSE03 that accounts for screened exchange. In contradiction to experimental observations, GGA predicts the fourfold coordinated zb and wz structures to be energetically favorable in comparison to the sixfold-coordinated rs geometry. The use of the hybrid functional HSE03 improves the energetic stability of the rs structure but still fails to determine the correct ground state of MnO. This deficiency can be overcome by applying the GGA+U method with U4 eV. The computed total energies are used to fit the nearest- and next-nearest-neighbor exchange coupling constants of a Heisenberg model Hamiltonian. Only for the GGA+U functional with U4 eV, the coupling constants as well as the resulting critical temperature for the magnetic phase transition are in agreement with measured quantities. Therefore, we conclude that an appropriate treatment of the correlation effects in MnO and similar compounds is necessary not only for the electronic but also for the structural properties.