We begin with a discussion of the Si vacancy (V Si ), since the electronic structure of this defect can be understood solely from the Mg 8 molecular orbitals. While the molecular orbitals themselves are the same as those surrounding the interstitial site (Figure ), their energies will be lower due to the attractive Madelung potential of the anionic vacancy site (Figure ). Additionally, while the molecular orbitals of the Mg 8 complex surrounding the interstitial site are unfilled, the molecular orbitals surrounding the vacant Si site are filled with 4 electrons, which fill the bottom singlet a 1g state with 2 electrons and the triplet t 1u states with 2 electrons. DFT calculations suggest that the charge state of V Si is primarily +2 when E F is in the band gap (Section 3.