Generate gamma center k-points mesh grid for GW calc,
        which is requested by GW calculation.

def kpoints(self):
        """
        Generate gamma center k-points mesh grid for GW calc,
        which is requested by GW calculation.
        """
        return Kpoints.automatic_density_by_vol(self.structure,
                                                self.reciprocal_density,
                                                force_gamma=True)