ABSTRACT
This chapter shows that the ionisation potentials and the electron affinities of clusters including a substitution atom have no direct relation with those of this atom. Big clusters are almost cubic, with interatomic distances very close to those of the crystal. The suppression of the central oxygen atom leads to a sharing of its negative charge between its six first neighbours and to a slight increase of the sphericity of the inner part of the cluster. A comparison between these clusters requires a common reference. In the absence of level in the bandgap, a neutral crystal has a full valence band and an empty conduction band. The Mulliken charges of the impurity, which remain approximatively constant in all the other clusters, show a decrease in these clusters, indicating that at least a fraction of the removed electron comes from the central atom.
