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In a recent Letter, Fujii et. al. [Phys. Rev. Lett. 125, 016001] predicted anew 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$4.6 timeshigher than in diamond($\sim$92 GPa), the hardest material known to date.Pentadiamond, however was not investigated. Motivated by this, we useddifferent approaches to estimate the elastic properties of pentadiamond withdifferent approximations based on Density Functional Theory. The tests werealso carried out on diamond and other known hard carbon allotropes forvalidation. Through tests, we found that the elastic constants and elasticmoduli of pentadiamond reported in Ref. [Phys. Rev. Lett. 125, 016001] areincorrect, and pentadiamond should be considered a non-auxetic soft carbonallotrope.

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