1. B. R. Eggen and M. I. Heggie, School of Chemistry and Molecular Sciences, University of Sussex at Brighton, Falmer, BN1 9QJ, UK.
2. G. Jungnickel, Institut für Physik, Theoretische Physik III, Technische Universität Chemnitz-Zwickau, D-09107 Chemnitz, Germany.
3. C. D. Latham and R. Jones, Department of Physics, University of Exeter, Exeter, EX4 4QL, UK.
4. P. R. Briddon, Department of Physics, University of Newcastle, Newcastle, NE1 7RU, UK.