The charge state of Mg Si ranges from +4 when E F is near the valence band to -2 when E F is near the conduction band (Section 3.2). This suggests that the partially-filled bonding t 1u states are located in the band gap of Mg 2 Si, so that the states are deprived of 4 electrons when E F is close to the valence band edge and supplied with 2 electrons when E F is near the conduction band edge. As evidenced by the DFT-calculated partial density of states (Figure ), peaks that are primarily composed of porbitals of the Mg Si defect atom exist in the electronic structure. These peaks correspond to the bonding t 1u states; the peaks are completely filled when 2 additional electrons are added (Mg -2 Si ) and empty when 4 electrons are taken away (Mg +4 Si ), indicating that the peaks can hold up to 6 electrons just like the bonding t 1u states. The charge density distributions of the bonding t 1u states (Figure ) are mostly localized to the Mg Si defect atom as opposed to being spread over the host atoms, indicating that the bonding t 1u states do not mix with the conduction/valence band states and are therefore localized states in the band gap.