1. Laboratory for Atomistic and Molecular Mechanics, Massachusetts Institute of Technology, Cambridge, MA 02139
2. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139
3. Department of Engineering Science, National Cheng Kung University, Tainan 701, Taiwan
4. Department of Biomedical Engineering, Tufts University, Medford, MA 02155
5. Department of Civil and Industrial Engineering, University of Pisa, 56122 Pisa, Italy
6. Center for Computational Science and Engineering, Schwarzman College of Computing, Massachusetts Institute of Technology, Cambridge, MA 02139
7. Center for Materials Science and Engineering, Cambridge, MA 02139