1. Department of Molecular and Integrative Physiology, Department of Biochemistry, UIUC Programs in Biophysics, Neuroscience, and Bioengineering, National Center for Supercomputing Applications, and Beckman Institute, University of Illinois, Urbana, Illinois 61801, Department of Physics, University of South Florida, Tampa, Florida 33620, and Department of Biological, Chemical, and Physical Science, Illinois Institute of Technology, Chicago, Illinois 60616