1. Chemical Center, Department of Theoretical Chemistry, Lund University, P.O. Box 124, S-221 00 Lund, Sweden, Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Center for Complex Molecular Systems and Biomolecules, Flemingovo nam. 2, 166 10 Praha 6, Czech Republic, and Department of Chemistry, Stanford University, Stanford, California 94305