Bonding character of hydrogen in aluminum clusters

Ab-initio calculations are performed to study interaction of hydrogen on aluminum clusters. The results show large binding energies for a single hydrogen atom on Al₇ and Al₁₃ and two hydrogen atoms on Al₆. The highest occupied-lowest unoccupied molecular orbital (HOMO-LUMO) gaps of Al₇ and Al₁₃ become dramatically large by absorbing hydrogen. These large binding energies and HOMO-LUMO gaps make Al₇H and Al₁₃H clusters magic(s). The atomic structures and binding energies of Al₆H₂ and Al₈H₂ are used to explain the reason why hydrogen is likely to be dissociated only on Al₆. These results are discussed in the light of available experimental data and are in very good agreement with them.