  We compare the behavior of bond lengths, cross sectional shape and bulk
modulus in equilibrium structure at ambient conditions and under hydrostatic
pressure of all the three kinds of uncapped single walled carbon nanotubes.
Results of our numerical calculations show that two bond lengths completely
describe the structure of achiral SWNT whereas only one bond length is required
to determine structure of chiral SWNT. In armchair tubes, one bond length is
found to be larger than that of graphitic value while in zigzag tubes one bond
length has a constant value. These bond lengths are very sensitive to tube
radius. In chiral tubes, the value of bond length is found to depend on the
chirality and slightly on the tube radius. Different responses of these bond
lengths are found on application of pressure. At some critical pressure, both
bond lengths become equal to each other in achiral tubes. An analysis regarding
the cross sectional shape of the nanotubes and its pressure dependence has also
been done. The shape transition, from circular to oval shape takes place. At
this transition, the behavior of bond lengths is found different and dependent
on the chirality of the tubes. Chiral tubes with chiral angle which is mid way
between zigzag and armchair tubes are found to have most prominent effects of
chirality. Thus we demonstrate that pressure is a useful probe to characterize
various kinds of carbon nanotubes.

Materials Science