[1] Univ Cambridge, Ctr Computat Chem, Univ Chem Lab, Cambridge CB2 1EW, England
[2] UCL, Dept Phys & Astron, London WC1E 6BT, England
[3] Open Univ, Dept Phys Sci, Milton Keynes MK7 6AA, Bucks, England
[4] STFC Daresbury Lab, Computat Sci & Engn Dept, Warrington WA4 4AD, Cheshire, England
[5] Max Born Inst, Div B, D-12489 Berlin, Germany