1. Department of Chemistry, University of Rome “La Sapienza”, P. le A. Moro 5, 00185 Roma, Italy
2. CASPUR, Inter-University Consortium for Supercomputing in Research, via dei Tizii 6b, 00185 Roma, Italy
3. Department of Chemistry, Swedish University of Agricultural Sciences, P.O. Box 7015, SE-750 07 Uppsala, Sweden