  We show that the key formula in the works [Ding, Zhu, and Berakdar, Phys.
Rev. B {\bf 79}, 045405 (2009); {\bf 84}, 115433 (2011); Ding, Zhu, Zhang, and
Berakdar, Phys. Rev. B {\bf 82}, 155143 (2010)] is invalid in the extended
graphene system they investigated. The correct formalism in the extended
infinite system is also presented in this comment.
