In the framework of the density functional theory, we study the energe
tics and geometry of three-dimensional solids made of single-shell car
bon nanotubes. Amongst the investigated forms, a hexagonal packing wit
h an interdistance of 3.14 Angstrom between the tubes is found to be t
he most stable. We find that the matching of atomic positions between
nearest-neighbour tubules is an important contribution to the intertub
ule cohesive energy, and we explore the consequences of this fact on p
acking of non-hexagonal tubules. The ab initio calculation of the elec
tronic properties of our most stable hexagonal packing reveals that th
e zero-gap feature of the isolated tubule could eventually be conserve
d in solid-state environment.