1. Contribution from the CASPUR, Supercomputing Consortium, via de' Tizii 6, 00185 Rome, Italy, Department of Chemistry and CNISM, University of Rome La Sapienza, Piazzale A. Moro 5, 00185 Rome, Italy, Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, and Institut fuer Chemie und Biochemie, Physikalische und Theoretische Chemie, Freie Universitaet Berlin, Takustrasse 3, 14195 Berlin, Germany