  Density functional theory with local density approximation for exchange and
correlation functional is used to tune the electronic band structure of
silicene monolayer. The cohesive energy of free standing monolayer is
increasing (decreasing) with external electric field (distortion). Electrons in
silicene behave like Dirac fermions, when the bond angle between the Si atoms
is larger than $\sim 102^{0}$. Large distortions destroy the electronic
structure of silicene and silicene is no longer a semi-metallic material, and
the distorted silicene acts like an $n$-doped system. Electric field opens a
band gap around $K$ point in the Brillouin zone, which increases with electric
field. The bond angle between the Si atoms is a key player to determine the
presence or absence of Dirac cones in silicene.
