  We present first-principles calculations of electronic properties of graphene
under uniaxial and isotropic strains, respectively. The semi-metallic nature is
shown to persist up to a very large uniaxial strain of 30% except a very narrow
strain range where a tiny energy gap opens. As the uniaxial strain increases
along a certain direction, the Fermi velocity parallel to it decreases quickly
and vanishes eventually, whereas the Fermi velocity perpendicular to it
increases by as much as 25%. Thus, the low energy properties with small
uniaxial strains can be described by the generalized Weyl's equation while
massless and massive electrons coexist with large ones. The work function is
also predicted to increase substantially as both the uniaxial and isotropic
strain increases. Hence, the homogeneous strain in graphene can be regarded as
the effective electronic scalar potential.
