1. Department of Computational Biology and Biophysics, Sandia National Laboratories, Albuquerque, New Mexico 87185, USA;
2. Current affiliation: Department of Physics, University of Vermont, Burlington, Vermont 05405, USA
3. Department of Chemical and Biomolecular Engineering, Tulane University, New Orleans, Louisiana 70118, USA
4. Current affiliation: Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, USA