1. Department of Mathematics and Informatics, Ecole Normale Supérieure, 45 Rue d'Ulm, 75005 Paris, France
2. School of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, U.K.
3. CEMI, Russian Academy of Sciences, Nakhimovskii pr. 47, 117418 Moscow, Russia