1. Institute of Physical and Theoretical Chemistry, University of Regensburg, D-93040 Regensburg, Germany, Institute of Macromolecular Chemistry, University of Bayreuth, D-95440 Bayreuth, Germany, DBCM/SBCe, C. E. de Saclay, F-91191 Gif sur Yvette Cedex, France, SCM, C. E. de Saclay, F-91191 Gif sur Yvette Cedex, France, and Department of Applied Mathematics, A. N. U. Canberra, Australia