1. Department of Computer Science, Cornell University, Ithaca, NY 14853, USA.
2. Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, USA.
3. Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
4. Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125, USA.