The geometries and electronic structures of three kinds of (5, 5) armchair carbon nanotubes with open or capped ends were studied by PM3 method. The results indicate, as the length of the tube increases, in addition to the periodical variation of the energy gap between the HOMO and LUMO, the geometries of tube also exhibit oscillation and can be divided into three groups. For the nanotubes belonged to the same group, their geometries and the electronic structures are very similar. For the cap of nanotube, it can be seen as the prolongation of the tube wall, therefore, the electronic structures of tube with capped ends are close to the tube with open ends. Furthermore, the surface states located at the ends of nanotube were also discussed in this paper.

Key words: SCALE EFFECT, BONDING, SURFACE STATES, ELECTRONIC STRUCTURE, QUANTUM CHEMISTRY