Set parameters in cell.in for a GW computation
        :param nv__band: number of valence bands to correct with GW
        :param nc_band: number of conduction bands to correct with GW
        :param n_iteration: number of iteration
        :param n_grid and dE_grid:: number of points and spacing in eV for correlation grid

def set_GW_options(self, nv_band=10, nc_band=10, n_iteration=5, n_grid=6,
                       dE_grid=0.5):
        """
        Set parameters in cell.in for a GW computation
        :param nv__band: number of valence bands to correct with GW
        :param nc_band: number of conduction bands to correct with GW
        :param n_iteration: number of iteration
        :param n_grid and dE_grid:: number of points and spacing in eV for correlation grid
        """

        self.GW_options.update(nv_corr=nv_band, nc_corr=nc_band,
                               nit_gw=n_iteration)
        self.correlation_grid.update(dE_grid=dE_grid, n_grid=n_grid)