  By combining ab initio all-electron localized orbital and pseudopotential
plane-wave approaches we report on calculations of the electron affinity (EA)
and the ionization potential (IP) of (5, 5) and (7, 0) single-wall carbon
nanotubes. The role played by finite-size effects and nanotube termination has
been analysed by comparing several hydrogen-passivated and not passivated
nanotube segments. The dependence of the EA and IP on both the quantum
confinement effect, due to the nanotube finite length, and the charge
accumulation on the edges, is studied in detail. Also, the EA and IP are
compared to the energies of the lowest unoccupied and highest occupied states,
respectively, upon increasing the nanotube length. We report a slow convergence
with respect to the number of atoms. The effect of nanotube packing in arrays
on the electronic properties is eventually elucidated as a function of the
intertube distance.
