1. Dept. of Mechanical Science and Engineering, Univ. of Illinois at Urbana-Champaign, Urbana, IL 61801.
2. Dept. of Mechanical Science and Engineering, Institute for Condensed Matter Theory and Beckman Institute, Univ. of Illinois at Urbana-Champaign, Urbana, IL 61801 (corresponding author).
3. Dept. of Mechanical Science and Engineering, National Center for Supercomputer Applications, Univ. of Illinois at Urbana-Champaign, Urbana, IL 61801.