HARTREE-FOCK PERTURBED-CLUSTER TREATMENT OF LOCAL DEFECTS IN CRYSTALS .4. THE CLUSTER ELECTROCHEMICAL POTENTIAL

This paper, the fourth in a series, discusses how to correct the formation energy of a defect, as evaluated by the program EMBED, to take account of electron charge transfer between the cluster and its surroundings. Arguments are first provided, based on perturbation theory, to justify expressing this correction in the form: DELTAE(Q(cor)) almost-equal-to muq(Mis). Here, q(Mis) is the ''missing charge'', that is, the charge that must be poured into the reservoir of the infinitely extended outer space in order to insure charge balance, and the coefficient mu is called the ''cluster electro-chemical potential''. Next, the ''v-technique'' for determining mu is described. It consists in adding a constant potential in the local space of the host perfect crystal, and looking for local energy changes associated with charge transfer between the local and the outer space. It is shown that, for small cluster sizes, mu depends on the size and chemical composition of the cluster. Finally, examples are provided which document the effectiveness of the present methodology, with reference to the cation and anion vacancy in a LiF (001) monolayer.