This defect is of great importance since it was proven to be the stablest defect obtained during ion implantation . The <100> split-interstitial is obtained by replacing one carbon atom by two carbon atoms, displaced along the  and  directions from the lattice site (see figure (5.3)). Each of these two atoms forming the interstitial pair have only three nearest neighbors, which belong to planes that are perpendicular one to another.
The wave functions of the two available electrons are p orbitals centered on the two carbon atoms forming the interstitial pair, and pointing out of the planes containing their three nearest neighbors. For orbitals oriented relative to each other by a dihedral angle , the interaction between the p orbitals is reduced by cos. Since the plane in consideration are perpendicular, there is no mixing between these two orbitals. This therefore gives rise to two degenerated states in the band gap, each occupied by one electron.
The electronic structure of the <100> split-interstitial was studied by Uhlmann et. al.  with a sophisticated version of the tight binding model, coupled with the density functional theory. The local density of electronic states calculated reveals the presence of two degenerated states above the valence band. However their positions relatively to the band gap were not considered by the authors. In the present research, we will also calculate the energy levels induced by this defect, using the tight binding model explained below. Particular attention wil be paid to the relative positions of the levels induced by this defect, compared to those created by vacancies.