1. Schlumberger Cambridge Research, High Cross, Madingley Road, Cambridge, CB3 0EL, UK
2. Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0EL, UK
3. Department of Physics, University of Oxford, 1 Keble Road, Oxford OX1 3NP, UK
4. Center for Computational Science, Boston University, 3 Cummington Street, Boston, Massachusetts 02215, USA