Mg charged defect (Figure ). The Si on Mg antisite defect (Si Mg ) interacts primarily with Si 4 as it occupies the Mg site, and we can therefore understand the electronic structure of the defect through interactions be- Mg ), which is a 2 electron acceptor. The molecular orbitals of the Si 4 complex surrounding a Mg site are composed of p-orbitals of the 4 Si atoms. The p-orbitals hybridize and split into an s-like a 1 state, two sets of p-like t 2 triplets, an e doublet, and a t 1 triplet (a). The shading represents the relative phases of the orbitals. The molecular orbital energies are in the valence band of Mg 2 Si (b). There are 22 electrons total when V Mg is created, which fill the molecular orbitals up to the t 1 state and partially fills the top t 2 state with 4 electrons. The -2 charge state of V Mg suggests that the t 2 states exist in the valence band, so that 2 additional electrons completely fill the t 2 states when E F is in the band gap. Rotating depictions of the Si 4 molecular orbitals are available as GIFs in a public repository. tween the atomic orbitals of the Si defect atom and the molecular orbitals of Si 4 (Figure ). Analogous to the Mg Si defect, the Madelung potential raises the atomic orbital energies of the Si atom (Figure ), since a cation site is repulsive for electrons. By symmetry, the s-orbital of the Si defect atom will only interact with the spherically-symmetric a 1 state of Si 4 , and the p-orbitals of the Si defect atom will only interact with the t 2 states of Si 4 . The 26 total electrons from Si 4 (22 electrons) and the Si defect atom (4 electrons) will completely fill the molecular orbitals of Si Mg up to and including the antibonding a * 1 state (Figure ). It turns out that the a * 1 state is a localized state residing in the band gap of Mg 2 Si. DFT calculations indicate that the charge state of Si Mg is +2 when E F is near the valence band and 0 when E F is near the conduction band (Section 3.2). A chemical explanation for this charge state transition is that the antibonding a * 1 state is in the band gap, such that the charge state is neutral when E F is above the a * 1 state (as in Figure ) and 2 electrons are depopulated from the a * 1 state when E F is near the valence band edge. The partial density of states of Si Mg in Mg 2 Si corroborates this interpretation, where a 2 electron state that is predominantly s-character is filled in Si 0 Mg and empty in Si +2 Mg (Figure ). Moreover, the charge density of the a * 1 state is primarily centered around the Si Mg defect site (Figure ) as opposed to being spread over regions far from the defect, indicating that the a * 1 state is a localized state that does not mix with either the conduction nor valence band states.