The charge state of a defect can also be understood using the same ideas of chemical bonding and molecular orbital theory that are widely recognized in molecules, as if the charged defect is an atom in a molecule with an associated formal charge. In AB, by removing a B atom from the crystal structure, the surrounding atoms interact and form new molecular orbital states. If E F is near the conduction band and the 2 electrons remain localized at the defect site, then this can be understood as the filling of a 2-electron molecular orbital state that exists below E F (Figure ). On the other hand, if E F is close to the valence band and the 2 electrons are delocalized from the defect site, than this can be interpreted as an unfilled molecular orbital state that exists above E F (Figure ).