1. Contribution from the Division of Computational Bioscience, Building 12A, Center for Information Technology, National Institutes of Health, Bethesda, Maryland 20892-5624, Laboratory of Chemical Physics, Building 5, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892-0520, Structural Biophysics Laboratory, National Cancer Institute, P.O. Box B, Frederick, Maryland 21702-1201, and Laboratory of Biophysical Chemistry, Building 50,...