1. Pritzker School of Molecular Engineering, University of Chicago, Chicago, IL 60637, USA.
2. Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR.
3. OpenEye Scientific Software, Inc., 9 Bisbee Court Suite D, Santa Fe, New Mexico 87508, USA.
4. Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA.