All calculations were performed with the Vienna Ab-initio Simulation Package (VASP) 36 using projector-augmented wave pseudopotentials (PAW) to approximate the core electrons. A plane wave basis set with a cutoff of 600 eV was chosen to represent the valence states. Calculations were performed on a gamma-centered Monkhorst-Pack grid with a minimal density of k-points in each orthogonal direction specified by the equation: 50 Å/(unit cell length). Energies were relaxed to a 10 -5 eV convergence threshold, while the forces on each atom were converged to 0.02 eV/Å. An effective Hubbard-U (U eff ) value was applied to the Mn 3d-electrons (described below) to better represent the semiconducting nature of the partially occupied Mn d-band.