Résumé
Floppy molecular systems are very challenging since a normal mode description is not possible for the description of large amplitude motion. In this talk, I will present a spectroscopic analysis of the accuracy of therecent ab initio potential of H2O-H2 for which several experimental data are now available. I willalso present the efficiency of a reduced dimensional model which can reproduce rovibrational boundstates with a good accuracy. In the last part, I will present the application of this two-body potentialfor the calculation of Translation-Rotation states of hydrogen in water clathrates.