Comparison of the optical properties of an aggregate made of 8 monomers calculated with DDA and with an equivalent using Mie theory using EMT (see Sect. 6 for details) for a-Sil_Fe,FeS (left panels), a-C (middle panels), and a-C:H (right panels) grains. From top to bottom panels: Q_abs, Q_sca, the ratio of the two previous quantities calculated with DDA vs. EMT, the albedo, and the asymmetry factor of the phase function. The solid lines are the DDA optical properties for a₀ = 0.1 (magenta) and 1 μm (green). The filled areas show the scatter of the dust properties for ten randomly chosen aggregate shapes (see Sect. 2.2). The crosses show the Mie calculations for the compact sphere of equivalent mass (a = a_V). The empty circles show the Mie calculations using optical constants obtained through EMT.