The nodal feature of the charge density, in which there is no charge between the central Si defect atom and the surrounding Si 4 complex (Figure ), indicates antibonding character consistent with the description of the a * 1 state. In fact, the charge density calculated using DFT agrees with our molecular orbital theory analysis. As drawn in Figure , we expect from molecular orbital theory that the a * 1 state consists of a spherical s-orbital at the center of the tetrahedral complex that is out-of-phase with the a 1 state of the Si 4 complex (Figure ). Since the orbitals (i.e., wave functions) are out-of-phase, we expect destructive interference between the s-orbital and the a 1 state of Si 4 . As a result, the wave function should be zero in the region between the defect atom and the vertices of the Si 4 complex, in agreement with the charge density profile shown in Figure .