For pt. I see ibid., vol.4, 3065 (1971). The changes in the electronic structure of diamond and silicon crystals when a substitutional impurity atom is present are investigated within the framework of the one electron extended Huckel method. In most of the studies reported a cluster of 41 atoms is used to simulate the crystal. For diamond. the electronic properties associated with having a silicon, nitrogen, boron or aluminium substitutional impurity at the centre of the cluster are investigated. Similar calculations for silicon with a carbon, boron, aluminium, nitrogen, oxygen or sulphur substitutional impurity are also discussed. A more detailed study of the nitrogen donor in diamond shows that the position of the impurity level in the band gap depends upon the size of the cluster chosen and the approximations made at the surface. It is concluded that such a simple one-electron treatment is inadequate for the quantitative study of the electronic properties of impurity ions in covalent crystals.