  The temperature dependences of the lifetimes of polycubanes C4+4nH8 with n =
2 - 5 up to their decomposition have been directly calculated using the
molecular dynamics method. It has been shown that the activation energy of
decomposition of these metastable clusters, in which the C-C bonds form an
angle of 90^0 that is not characteristic of carbon systems, rapidly decreases
with an increase in n due to the lowering of the energy barrier that prevents
the decomposition of the clusters. This has cast some doubt on the recently
made suggestion that there exist nanotubes (n >> 1) with a square cross
section. Nonetheless, the stability of bicubane (n = 2) and tricubane (n = 3)
has proved to be sufficient for their existence at the liquid-nitrogen
temperature.
