1. Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Federal Republic of Germany
2. RIKEN Center for Computational Science, 7-1-26 Minatojima-minami, Chuo Kobe 650-0047, Japan
3. Yusuf Hamied Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom