1. INFM-CNR DEMOCRITOS National Simulation Center and Dipartimento di Fisica “G. Galilei”, Università di Padova, via Marzolo 8, I-35131 Padova, Italy, Departament ECM, Facultat de Física, and IN2UB, Universitat de Barcelona. Diagonal 647, 08028 Barcelona, Spain, and Departament of Chemistry and Physics, University of Virginia, 22904-4319 Charlottesville, Virginia