Theoretical study of molecular hydrogen clusters

Hydrogen clusters are formed by packing H₂ molecules. A structural characterization of (H₂)_N clusters up to N=35 has been carried out at zero temperature by using density functional theory. The binding between the hydrogen molecules is very weak and the cluster growth reminds that of the inert gas clusters. An icosahedron is obtained for (H₂)₁₃. For clusters larger than (H₂)₁₃ several growth models have been compared. The binding energy indicates specially stable clusters for some particular sizes. The magic numbers can be related to Raman spectroscopy experiments, where the intensity of the Raman signal serves to assign enhanced abundance to clusters with N≈13,32,55, which coincide with some of the most stable clusters obtained in the present study. In addition, comparison of theory and experiment suggests that clusters with N smaller than 27 are liquid. The photoabsorption spectra have been calculated using time-dependent density functional theory. Those spectra can be interpreted as a widening of the absorption peaks of the H₂ molecule due to the various environments experienced by different molecules in the same cluster.