  In a recent Letter [1] Y. Fujii et al. proposed a new carbon allotrope and
claimed its extremely high Young's and shear moduli of 1691 and 1113 GPa,
respectively, surpassing those of diamond by 1.5 and 2 times! The authors also
claimed unique negative averaged Poisson's ratio of -0.24, bulk modulus of 381
GPa and record value of the sound speed 28.7 km/s for this carbon modification.
Here we show by two independent computations that these values were obtained as
a result of errors. The new hypothetical allotrope has in fact quite moderate
elastic moduli, unremarkable for a carbon structure with similar density: bulk
modulus of 250 GPa, shear modulus of 170 GPa,Young's modulus of 417 GPa,
positive Poisson's ratio of 0.22.
