Comment on “Increase in specific heat and possible hindered rotation of interstitial ${\text{C}}_2$ molecules in neutron-irradiated graphite”

Iwata and Watanabe’s model for the observed low-temperature specific heat of neutron-irradiated graphite [T. Iwata and M. Watanabe, Phys. Rev. B 81, 014105 (2010)] assumes that self-interstitial atoms exist as clusters of nearly free ${\text{C}}_2$ molecules. We suggest that their hypothesis is not supported by other experiments and theory, including our own calculations. Not only is it inconsistent with the long-known kinetics of interstitial prismatic dislocation loop formation, density-functional theory shows that the di-interstitial is covalently bonded to the host crystal. In such calculations no prior assumptions are made about the nature of the bonding, covalent or otherwise.