As shown in Figure , a point defect that occupies the interstitial site is cubically coordinated to 8 Mg atoms (Mg 8 for brevity). As a result, we would expect the atomic orbitals of the interstitial atom to interact primarily with the molecular orbitals of the Mg 8 complex, and we therefore begin by drawing the Mg 8 molecular orbitals as one would expect from symmetry. The Mg 8 molecular orbitals near the conduction band edge are primarily constructed from the s-orbitals of the 8 Mg atoms. As shown in Figure , the Mg 8 states are split by symmetry into a singlet a 1g state, triplet t 1u states, triplet t 2g states, and a singlet a 2u state, labeled according to the O h point group of the Mg 8 complex. The molecular orbitals of the Mg 8 complex are determined using symmetryadapted linear combinations of atomic orbitals. Certain interactions between the interstitial atom and the Mg 8 states are allowed by symmetry. For example, we notice in Figure that the singlet a 1g state is composed of Mg-s orbitals that are all in phase, indicating that the a 1g state is spherically-symmetric. Accordingly, the a 1g state can only interact with orbitals of an interstitial atom that are also spherically-symmetric, e.g., an sorbital. The triplet t 1u states all share a feature where the Mg-s orbitals are in phase on the sides of the Mg 8 cube, but out of phase between opposing faces. The t 1u states therefore only interact with the p-orbitals of an interstitial atom.