  Stability of recently predicted cage-like carbon cluster C$_8$ (prismane)
against its transformations to structures lying at lower energies is studied
theoretically. The smallest energy barrier inhibiting prismane transformation
is shown to be 0.44 eV, in qualitative agreement with the value of activation
energy for prismane decay calculated by molecular dynamics simulations. A
rather high value of the barrier suggests the possibility of experimental
observation of this cluster at ambient pressure.
