1. for a review see: de Heer, W.A., Rev. Mod. Phys. 1993
2. Harbich, W., Fedrigo, S., Meyer, F., Lindsay, D.M., Lignieres, J., Rivoal, J. C. and Kreisle, D., J. Chem. Phys.93 (1990) 8535.
3. Charl�, K. P., Schulze, W., Winter, B., Z. Phys. D12 (1991) 471.
4. Tiggesb�umker, J., K�ller, L., Lutz, H.O. Meiwes Broer, K. H. ?From clusters to crystals?, Eds. P. Jena, S.N. Khanna, B.K. Rao (Kluwer Academic publishers, Boston 1992), Tiggesb�umker, J., K�ller, L., Lutz, H.O. Meiwes Broer, K. H., Chem. Phys. Lett.190 (1992) 42. and private comm.
5. Harbich, W., Fedrigo, S., Buttet, J., Chem. Phys. Lett.195, 613, (1992)