The Vickers hardness values evaluated using the four contemporary hardness models are summarized in Table . A clear tendency of a decrease in hardness in the C 5 -C 6 -C 7 series due to the increasing openness of the crystal structures is observed for all four models. Since it has been shown earlier that the thermodynamic model is the most reliable in the case of the superhard compounds of light elements and shows perfect agreement with the available experimental data , it is obvious that the hardness values calculated within the empirical models are strongly underestimated. The Oganov-Lyakhov model also gives underestimated (by 10-20%) values.