The molecular orbitals resulting from interactions between an interstitial defect atom and the surrounding Mg 8 complex are shown in Figure . The bonding a 1g state exhibits an in-phase interaction between the s-orbital of the defect atom and the a 1g state of the Mg 8 complex, whereas the antibonding a * 1g state consists of an out-of-phase interaction. Similarly, the bonding t 1u state is composed of in-phase interactions between the p-orbitals of the interstitial defect atom and the t 1u state of the Mg 8 complex, whereas the antibonding t * 1u state exhibits out-of-phase interactions between the p-orbitals and the t 1u state of Mg 8 . As the t 2g and a 2u states of the Mg 8 complex do not interact with the s-and p-orbitals of the interstitial defect atom, we do not expect any orbital contribution from the defect atom to those states. i ), both of which are 2 electron donors. For Mg i , the spherically-symmetric a 1g state of the Mg 8 complex (a) hybridizes with the s-orbital of the interstitial atom (c), forming the bonding a 1g and antibonding a * 1g states (b). The +2 charge state of Mg i suggests that the bonding a 1g state is located in the conduction band (b), such that the state is depopulated by 2 electrons when E F is in the band gap. Similarly, for Si i , the a 1g state of the Mg 8 complex (d) hybridizes with the s-orbital of Si (f) to form the bonding a 1g and antibonding a * 1g states (e), and the t 1u states of Mg 8 (d) interact with the p-orbitals of Si (f) to form the bonding t 1u and antibonding t * 1u states (e). If the bonding t 1u state exists in the conduction band and the bonding a 1g state resides in the valence band, then Si i has a +2 charge state due to the removal of 2 electrons from the bonding t 1u state when E F is in the band gap.