1. Mathematical and Statistical Computing Laboratory, Division of Computational Bioscience, Center for Information Technology, National Institutes of Health, Bethesda, Maryland 20819, USA
2. Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20819, USA
3. Department of Physics and Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida 32306, USA