Dependence of optical properties on grain shape (left) and size (right) for silicate grains made of aSil-1/a-C^2.5nm materials (top) and carbonaceous grains with a-CH/a-C^10nm composition (bottom). Each block of figures presents the total absorption and scattering efficiencies as solid and dashed lines (upper figure), respectively, and the polarisation efficiency Q_pol = (β_2,ext − β_1,ext)/2 (lower figure). For comparison, Q_pol for THEMIS I oblate silicates with e = 1.3 and variable size are shown in the lower figure of the top right block.