This initial symmetry analysis of the molecular orbitals of the Mg 8 complex guides our understanding of the allowed chemical interactions between an interstitial defect atom and the host atoms near the interstitial site (Figure ). For example, the sorbital of the Mg interstitial (Mg i ) interacts with the a 1g state of the Mg 8 complex and forms bonding and antibonding states (labeled a 1g and a * 1g respectively in Figure ), just as in molecules when two atoms form a bond. Similarly, the p-orbitals of Mg i interact with the triplet t 1u states of Mg 8 , forming the bonding t 1u and antibonding t * 1u states. The t 2g and a 2u states of Mg 8 however do not interact with any atomic orbital of Mg i due to symmetry restrictions. A similar analysis can be carried out for the Si interstitial (Si i ), where the s-and p-orbitals of Si i interact with the a 1g and t 1u states of Mg 8 , respectively (Figure ). However, since the energies of the Si atomic orbitals are lower than those of Mg, we would expect the molecular orbital energies of Si i to be lower than those of Mg i .