  In order to assist the search of new superconductors in iron selenide
materials by intercalation, we calculate the crystal and electronic structures
of MgOFeSe using the first-principles density functional theory. MgOFeSe is
isotructural to the parent compound of iron pnictide superconductor LaOFeAs. In
LaOFeAs, the anion O$^{2-}$ is located at the center of each LaO tetrahedra.
But for MgOFeSe, we find that the crystal structure with the cation Mg$^{2+}$
as the tetrahedral center in the MgO layer is energetically more stable. The
low energy band structures around the Fermi surfaces of MgOFeSe are contributed
mainly by Fe 3$d$ orbitals. The ground state of MgOFeSe is collinearly
antiferromagnetically ordered. The height of Se atoms above the Fe-Fe layer is
about 1.38 \AA, which is close to the height of As from the Fe-Fe layer in the
iron pnictide superconductors with optimal superconducting transition
temperatures.
