Thermal Stability of C4+4nH8 Polycubanes
  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. 
M. M. Maslov, A. I. Podlivaev, and L. A. Openov
cond-mat.other physics.atm-clus