1. Department of Chemistry, University of Massachusetts Amherst, 710 North Pleasant Street, Amherst, Massachusetts 01003, United States
2. Department of Physics, University of Massachusetts Amherst, 666 North Pleasant Street, Amherst, Massachusetts 01003, United States
3. Computer Science and Mathematics Division, and Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States