1. Drug Discovery Program, Institute of Cognitive and Computational Science, Georgetown University Medical Center, 3970 Reservoir Road, NW, Washington, DC 20007-2197, Department of Pharmacology and Toxicology, University of Texas Medical Branch, Galveston, Texas 77555-1031, and Laboratory for the Structure of Matter, Code 6030, Naval Research Laboratory, 4555 Overlook, SW, Washington, DC 20275-5000.