In their first-principles calculations of the electronic band structure of graphene under uniaxial strain, Gui, Li, and Zhong [Phys. Rev. B 78, 075435 (2008)] have found opening of band gaps at the Fermi level. This finding is in conflict with the tight-binding description of graphene which is closed gap for small strains. In this Comment, we present first-principles calculations which refute the claim that strain opens band gaps in graphene. 73.61.Wp, 72.80.Rj Gui et al. 1 have used first-principles calculations to investigate the effect of planar strain on the electronic band structure of graphene, and have found opening of band gaps at the Fermi level resulting from arbitrarily small uniaxial strains, applied parallel or perpendicular to the C-C bonds. However, tight-binding (TB) model on the honeycomb lattice with different nearest-neighbor hoppings in the three directions has been rigorously shown to be closed gap as long as the hoppings satisfy the triangle inequality. 2,3 The closed-gap TB model, which contains zero modes, has been further developed recently to include the effects of magnetic fields 4,5 and corrugations in graphene. 6 The discrepancy between the firstprinciples calculations of Ref. 1 and TB model has already been discussed 7 but the reason has not been identified. One suggested explanation 7 is that this band gap opening is an artifact of density-functional theory (DFT) calculations. However, the possibility that the nearest-neighbor TB model is an incomplete description has not been ruled out. 7,8 We remind that the DFT methods essentially solve singleparticle Schrödinger equations (Kohn-Sham equations) for effective potentials based on the underlying lattice, and the TB model solves the same problem in a simplified approximation. Therefore, it seems unlikely that qualitative differences exist between DFT and TB band structures. We also see indications of possible error in Ref. 1. First, Figs. 3(c) and 5(c) of Ref. 1 show a peculiar peak whose underlying cause is not explained. Second, an energy gap is incorrectly ascribed to the TB band structure which is then plotted in Fig. 4 of Ref. 1 with large symbols that hide the important band crossing.In this Comment, we check directly the first-principles calculations of Ref. 1 by one of the available DFT codes. We used the QUANTUM-ESPRESSO 9 package based on the pseudopotential plane-wave method. We obtained the pseudopotential C.pw91-van ak.UPF also from Ref. 9 and used a kinetic-energy cutoff of 40 Ry, a Monkhorst-Pack k-point mesh of 21 × 21 × 1, and a vacuum separation of 20.5Å along the c axis. We chose these parameters as close as possible to those of Ref. 1 for a more meaningful comparison. 10 First, we determined the equilibrium lattice constant of graphene in the absence of strain. We found a value of a = 2.464Å, defined in Fig. 1(a), which is not significantly different from the 2.4669Å found in Ref. 1. We then made calculations on graphene under uniaxial strain for two special cases, for which Ref. 1 has found maximum values...
