1. Center for Theoretical Biological Physics, Rice University, Houston, TX 77005, USA
2. Theoretical Biology and Biophysics Group, Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
3. New Mexico Consortium, Los Alamos, NM 87544, USA
4. Departments of Physics and Astronomy, Chemistry, and Biosciences, Rice University, Houston, TX 77005, USA