Abstract
This work is concerned with the application of EHT (extended Hückel Theory) to a diamond crystal. It is first shown in simple cases how overlap integrals can give rise to a repulsive part in the total energy. Approximate forms of this repulsive energy are given in view of their use in more complex systems. Application to diamond is made, giving a simple analytic form for the total energy, which shows that the minimum is mainly due (in EHT) to the repulsive energy provided by the overlap between adjacent bonding orbitals