  The Kekule structure of the ground state of (5,5) armchair carbon nanotube is
revealed by semiempirical molecular orbital calculations. This structure has
bonds with two different bond lengths, differing by 0.003 nm. The ground state
has tripled (compared to undistorted case) translational period due to Peierls
distortions. Two first order structural phase transitions controlled by the
tension are predicted at zero temperature. These transitions correspond to 5%
and 13% elongations of a uniaxially deformed (5,5) nanotube. The narrow gap
semiconductor to metal transition is predicted at 5% elongation of the
nanotube.
