  An ab initio calculations have been carried out for examining the curvature
effect of small diameter hexagonal boron nanotubes. The considered
conformations of boron nanotubes are namely armchair (3,3), zigzag (5,0) and
chiral (4,2), and consist of 12, 20, and 56 atoms, respectively. The strain
energy is evaluated in order to examine the curvature effect. It is found that
the strain energy of hexagonal BNT strongly depends upon the radius, whereas
the strain energy of triangular BNTs depends on both radius and chirality.
