G. Martinez^* and Marvin L. Cohen^†,‡

• Laboratoire de Physique des Solides, Université Pierre et Marie Curie, 75 Paris 5°, France

M. Schlüter^†

• Department of Physics, University of California, Berkeley, California 94720, and Inorganic Materials Research Division, Lawrence Berkeley Laboratory, Berkeley, California 94720

• ^*Work supported in part by the Délégation Générale pour la Recherche Scientifique et Technique under Contract No. 70-02.181
• ^†Work supported in part by National Science Foundation Grant No. DMR72-03206-A02.
• ^‡Permanent address: Department of Physics, University of California, Berkeley, Calif. 94720.