While the molecular orbital energies of Mg i and Si i in relation to the conduction and valence bands of Mg 2 Si (Figures ) are certainly unclear from this analysis, symmetry provides valuable insight towards the degeneracies of the molecular orbitals and the possible charge states of the defects. For Mg 0 i , the bonding a 1g state is filled with 2 electrons, due to the 2 valence electrons introduced by the interstitial Mg atom. As we will see in Section 3.2, DFT calculations indicate that Mg i has a +2 charge state when E F is in the band gap. This suggests that the bonding a 1g state is located in the conduction band (Figure ) such that the 2 electrons are delocalized from the Mg i defect when E F is in the band gap. For Si 0 i , the bonding a 1g and t 1u states are both filled with 2 electrons each, as the interstitial Si atom introduces four valence electrons. DFT calculations indicate that Si i has a +2 charge state when E F is in the band gap (Section 3.2). A chemical interpretation consistent with this observation is that the bonding a 1g state is located in the valence band while the bonding t 1u state is in the conduction band, such that the Si +2 i charged defect forms from the delocalization of 2 electrons from the t 1u state (Figure ).