Geometries and energies for H3+ (H2)_n clusters (n=0,â¦, 11) have been calculated using standard ab-initio methods. Up to the clusters with n=6, three different Pople basis sets (DZ, TZ, TZP) have been used in the calculations. For larger cluster sizes, the calculations have been carried out with one basis set (DZ) using the HF/CISD method. We discuss here only the nice counterplay of polarization effects between the central H3+ ion and the adsorbed H2 molecules, which naturally governs both the geometric structure and energy of the clusters.