1. Department of Chemistry, University of Reading , Reading, RG6 2AD, England
2. University of London Computer Centre , 20 Guilford Street, London, WC1, England
3. Laboratoire de Spectroscopie Moléculaire, Institut de Physique Corpusculaire, Université de Louvain , Chemin du Cyclotron 2, B1348, Louvain-la-Neuve, Belgium