  Compressed hydrogen passes through a series of layered structures in which
the layers can be viewed as distorted graphene sheets. The electronic
structures of these layered structures can be understood by studying simple
model systems- an ideal single hydrogen graphene sheet and three-dimensional
model lattices consisting of such sheets. The energetically stable structures
result from structural distortions of model graphene-based systems due to
electronic instabilities towards Peierls or other distortions associated with
the opening of a band gap. Two factors play crucial roles in the metallization
of compressed hydrogen: (i) crossing of conduction and valence bands in
hexagonal or graphene-like layers due to topology and (ii) formation of bonding
states with $2p_z$ $\pi$ character.
