1. K. T. Thomson, Department of Chemical Engineering and Materials Science, and Minnesota Supercomputer Institute, University of Minnesota, Minneapolis, MN 55455, USA.
2. R. M. Wentzcovitch, Department of Chemical Engineering and Materials Science, and Minnesota Supercomputer Institute, University of Minnesota, Minneapolis, MN 55455, USA, and Instituto de Fisica, Universidade de Sao Paulo, CP 66318, 0 5389-970, Sao Paulo, SP, Brazil.
3. M. S. T. Bukowinski, Department of Geology and Geophysics, University of California, Berkeley, CA 94720, USA.