Defects that occupy the Mg site in Mg 2 Si are tetrahedrally coordinated to 4 Si atoms (Figure ), which we refer to collectively as the "Si 4 complex" for brevity. The electronic structure of the Mg vacancy (V Mg ) can be understood from the molecular orbitals of the Si 4 complex. The energies of the molecular orbitals constructed from the Si-p states are split into a singlet a 1 state, two sets of triplet t 2 states, doublet e states, and triplet t 1 states (Figure ). The molecular orbitals are determined using symmetryadapted linear combinations of atomic orbitals and labeled by symmetry according to the T d point group of the Si 4 complex (derivation shown in the Supplementary Information). Normally in the Mg 2 Si solid, there are two Mg cations that donate 2 valence electrons each to one Si anion, so that each ion has a closed shell electron configuration. The Si 4 molecular orbitals are similarly filled by valence electrons from 8 Mg atoms in a perfect, undefected crystal. However, by removing a Mg atom to create the V Mg defect, the number of electrons filling the Si 4 states changes. In total there are (2 p-orbital valence electrons per Si) × (4 Si atoms) + (2 s-orbital valence electrons per Mg) × (7 Mg atoms) = 22 electrons that fill the Si 4 states, which would leave the high-energy t 2 states deficient of 2 electrons. When E F is in the band gap, V Mg has a -2 charge state (Section 3.2). It is therefore reasonable to interpret the -2 charge state as a filling of the t 2 states by 2 additional electrons from the electron reservoir, forming the V -2