GGA and GGA+U calculations of the ground state of SrMnO3

A very recent experimental report suggests the ground state of SrMnO3 is orthorhombic with space group C2221. I perform spin-polarized generalized gradient approximation (GGA) calculations with/without on-site Coulomb interaction included (GGA+U) for this ground state of SrMnO3. In contrast to the latest theoretical report of cubic and hexagonal SrMnO3 where GGA method is considered to be able to reproduce the insulating behavior of SrMnO3, I find that for the orthorhombic SrMnO3, GGA method is not enough to reproduce its insulating behavior. When the on-site Coulomb interaction is included by means of GGA+U, the insulating character is recovered. The results show that correlation interaction plays an important role in the ground state of SrMnO3.