1. IBM Research Europe, Hartree Centre, SciTech Daresbury, Warrington, Cheshire WA4 4AD, UK
2. University of Edinburgh, School of Mathematics, Bayes Centre, 47 Potterrow, Edinburgh EH8 9BT, UK
3. University of Oxford, Physical and Theoretical Chemistry Laboratory, Oxford, UK
4. IBM Research, IBM Almaden Research Center, San Jose, CA 95120, USA
5. IBM Research, IBM T. J. Watson Research Center, Yorktown Heights, New York 10598, USA