Geometric optimization was performed with a convergence criterion of 10 -2 eV/Å for the force on atoms. A kinetic energy cutoff of 700 eV was used for Born effective charge calculations and 500 eV for all other calculations. All calculations containing Mn were done with DFT+U method with a U parameter of 3.9 eV applied to Mn. All Mn3O4 calculations were initialized in a ferrimagnetic state; slab calculations tended to relax to different magnetic states due to changes in coordination. All crystal structures can be found at .