A comparison of techniques for embedding defect cluster calculations

Cluster calculations have proved one of the most fruitful means of extracting useful information about solid-state defect properties at finite computational expense, but large clusters are necessary to obtain reliable results and uncertainty always remains concerning the effects of edge states, with ad hoc boundary conditions such as the saturation of the surface with hydrogen atoms being frequently employed. Recently methods have been developed for embedding the cluster in the perfect crystal so that the solution obtained from the cluster calculation is the same as would have been obtained from a full calculation on the whole defective solid. Here the relationships of these methods with each other and with perturbed crystal methods which modify the perfect crystal solution to account for the presence of the defect are explored. It is shown that the embedding potential technique is preferable and is equivalent to methods that have been used in other fields.This paper was presented at the International Conference on The Impact of Supercomputers on Chemistry, held at the University of London, London, UK, 13–16 April 1987