Standard dust model size distributions. a-C-coated (E_g = 0.1 eV, mantle depth 5 nm) amorphous forsterite-type silicate grains with ~70% of the available iron in metallic, nano-particle inclusions (triple dot-dashed). Core/mantle, a-C:H/a-C (E_g = 2.5/0.1 eV) particles: large, log-normal (dotted) and small, power-law with an exponential tail (dash-dotted). The grey line shows the overall a-C(:H) grain size distribution, with physically-continuous properties, which is only separated into “large” and “small” components for modelling convenience.