We investigate the conformations and electronic structure behavior of endoliedrally confined hydrogen molecules using density functional theory. An H-60 cavity is used to imprison 13H(2) molecules. The variation of the confinement cage radius leads to different compression rates, with the corresponding evolution of the molecular structure, energy, electron density, Mulliken populations, HOMO-LUMO gap and ionization potential. At low pressures our results compare favorably with measurements on the crystal. At moderate pressures the hydrogen molecules self-assemble into a stable cluster. In the high pressure range molecular dissociation occurs due to a high population in an antibonding orbital. Our results are of interest in the scientific challenge to achieve the metallic state of hydrogen under pressure. (c) 2005 Elsevier B.V. All rights reserved.