For Ga and In, 3d and 4d-electrons respectively were treated as valence electrons. To model the surface, a slab of each respective 13N compound was made from an optimized unit cell. First the unit cell was reoriented to make a conventional cell with axis along the [101 ̅ 0] and direction, containing 4 primitive cells. The conventional cell was then repeated in the [0001] direction and padded with vacuum. The number of layers and vacuum separation as well as cutoff energy and number of k-points were checked to be converged with respect to the surface energy. Dangling bonds on the bottom of the slab were passivated by hydrogen atoms to limit artificial charges across the slab and vacuum padding.