1. Istituto per le Applicazioni del Calcolo CNR, Via dei Taurini, 19, 00185 Rome, Italy
2. School of Engineering and Applied Sciences, Harvard University, McKay 517, Cambridge, Massachusetts 02138, USA
3. Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA
4. Center for Life Nanoscience at la Sapienza, Istituto Italiano di Tecnologia, Viale Regina Elena, 295, I/00161 Rome, Italy
5. Institute for Applied Computational Science, Harvard John A. Paulson School of Engineering and Applied Sciences, Cambridge, Massachusetts 02138, USA