title: Comment on "Hardness of covalent and ionic crystals: first-principle calculations".
authors.affiliation: State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China.
authors.forename: Z Y
authors.initials: ZY
authors.lastname: Liu
authors.forename: Xiaoju
authors.initials: X
authors.lastname: Guo
authors.forename: Julong
authors.initials: J
authors.lastname: He
authors.forename: Dongli
authors.initials: D
authors.lastname: Yu
authors.forename: Yongjun
authors.initials: Y
authors.lastname: Tian