[1] Univ Roma La Sapienza, Dipartimento Fis, I-00185 Rome, Italy
[2] CNISM, I-00185 Rome, Italy
[3] Univ Coll Dublin, Sch Phys, EMSC, Dublin 2, Ireland
[4] CSIC, Inst Quim Fis Rocasolano, E-28006 Madrid, Spain
[5] Univ Zaragoza, Inst Biocomputac & Fis Sistemas Complejos BIFI, Zaragoza 50018, Spain
[6] Univ Zaragoza, Dept Fis Teor, E-50009 Zaragoza, Spain