Spectral opacities for homogeneous spherical dust grains taken from Semenov et al. (2003) for five different temperature ranges. The grey colour bands represent the five different frequency groups that were used in the multigroup simulation. The dotted vertical lines indicate the additional splitting of groups 2 and 3 in the case of the 12-group simulation. The dashed line represents the Planck function for a black body with a typical temperature of 1000 K normalised to arbitrary units. The numbers in the lower part of the figure identify the different groups used in the 12-group simulation (see Sect. 3).