Interaction of a hydrogen molecule with a water cage (H_2O)_8

We study the stability of a hydrogen molecule H-2 interacting with an octamer water cage (H2O)(8), both in the D-2d and in the S-4 configurations. The H-2 molecule was allowed to approach the water cage along its main symmetry directions, resulting in physisorbed final configurations, with binding energy 0.01 - 0.06 eV, depending on the starting geometry and the level of approximation used. The vibrational spectra of all stable configurations show an increase of both IR intensity and Raman activity below 300 cm(-1), due to dimer - octamer interaction. Finally, we have also considered an H-2 dimer inside a water octamer cage. In this case, we confirm a sizeable blue-shift of the H-2 frequency, as has been recently found also for the H-2 center dot (H2O)(12) cluster.