1. Department of Chemistry, Rice University, Houston, Texas 77005, USA
2. Center for Theoretical Biological Physics, Rice University, Houston, Texas 77005, USA
3. Mathematical and Statistical Computing Laboratory, Office of Intramural Research, Center for Information Technology, National Institutes of Health, Bethesda, Maryland 20892, USA
4. Department of Chemical and Biomolecular Engineering, Rice University, Houston, Texas 77005, USA
5. Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA