  Based on the helical and rotational symmetries and Tersoff-Brenner potential
with couple of modified parameters, we investigate the variation of bond
length/lengths in equilibrium structure due to tube length as well as due to
applied hydrostatic pressure for a series of high symmetry armchair (n,n)
single-wall nanotubes having different radii. Assuming that two different bond
lengths dictate the tube geometry, these are monitored as a function of radius.
It turns out that one of these bond lengths is greater than bond length of
graphite whereas other one was less than it. These deviations from graphite
value appear to be related to the curvature-induced rehybridization of the
carbon orbitals. Lengths are found to have very important effect on the values
of both bond lengths. The results under hydrostatic pressure indicate many
linear behaviors having different slopes in the values of bond lengths with
increasing pressure leading to a pressure induced-phase transition. This
behaviour is strongly dependent on the tube radius. We also calculate the bulk
moduls for this structure which reflects clearly this behavior of armchair
nanotubes and thus predicts mechanical resilience of nanotubes.
