1. Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, Computational Science Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, Korea, NIST Center for Neutron Research, Gaithersburg, Maryland 20899-8562, Department of Material Science and Engineering, University of Maryland, College Park, Maryland 20742, and Department of Chemistry and CSGI, University of Florence, Sesto F.no, Florence, I-50019