  In a recent Letter, Fujii et. al. [Phys. Rev. Lett. 125, 016001] predicted a
new carbon allotrope, pentadiamond, with remarkable mechanical properties:
elastic moduli comparable or larger than diamond, and negative Poisson's ratio
$\mu$ = -0.241. The estimated Vicker's hardness is 422 GPa, $\sim$4.6 times
higher than in diamond($\sim$92 GPa), the hardest material known to date.
Pentadiamond, however was not investigated. Motivated by this, we used
different approaches to estimate the elastic properties of pentadiamond with
different approximations based on Density Functional Theory. The tests were
also carried out on diamond and other known hard carbon allotropes for
validation. Through tests, we found that the elastic constants and elastic
moduli of pentadiamond reported in Ref. [Phys. Rev. Lett. 125, 016001] are
incorrect, and pentadiamond should be considered a non-auxetic soft carbon
allotrope.
