1. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
2. Centre for Modeling and Simulation, S P Pune University, Ganeshkhind, 411007 Pune, India
3. Department of Physics, University of California, Berkeley, Berkeley, California 94720, USA
4. LITEN, CEA-Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France