1. Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford University, Oxford, OX1 3QZ, U.K.
2. FDA Biophysics Laboratory, Center for Biologics Evaluation and Research, 8800 Rockville Pike, Bethesda, Maryland 20892
3. Division of Computer Research & Technology, National Institutes of Health, Bethesda, Maryland 20892
4. Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden