1. Center for Life Nano Science@La Sapienza, Istituto Italiano di Tecnologia, 00161 Roma, Italy
2. Istituto per le Applicazioni del Calcolo CNR, via dei Taurini 19, Rome, Italy
3. School of Engineering and Applied Sciences, Harvard University, McKay 517, Cambridge, Massachusetts 02138, USA
4. Université degli Studi di Milano, via Celoria 16, 20133 Milano, Italy
5. Institute for Applied Computational Science, John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
6. Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA