Renata M. Wentzcovitch^*

• Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, United Kingdom

Werner W. Schulz^† and Philip B. Allen^‡

• Department of Physics, State University of New York at Stony Brook, Stony Brook, New York 11794-3800

• ^*Present address: Dept. of Chemical Engineering and Materials Science, Univ. of Minnesota, Minneapolis, MN 55455.
• ^†Present address: Institute of Physics and Astronomy, Århus University, DK-8000 Århus, Denmark.
• ^‡Present address: Institut Romand de Recherche Numérique en Physique des Matériaux (IRRMA), IN Ecublens, CH-1015 Lausanne, Switzerland.