Comment on "Gap opening in graphene by shear strain"

G. Cocco, E. Cadelano, and L. Colombo [Phys. Rev. B 81, 241412(R) (2010)] have suggested that combinations of shear and uniaxial strain can be used to open a band gap in graphene at much lower levels of strain than with the application of unaxial strain alone. They employed a unit cell of graphene in their studies and applied the Cauchy-Born rule to model external strain. Consequently, an important aspect of the mechanical behavior of membranes, namely buckling and wrinkling under external strain, and the attendant coupling with electronic structure was ignored in their analysis. Upon doing so, the apparent band gap that appears in the range of 15-20% shear strain under the Cauchy-Born assumption is shown to vanish. The gapless spectrum of graphene is found to persist under large shear strains as well as large combinations of shear and uniaxial strain.