Panel a: astronomical UV bump observed on the average Galactic extinction curve (Fitzpatrick & Massa 2007). Panel b: experimental spectrum of soot from Gavilan et al. (2017) (C/O ratio of 1.05 and a HAB of 22 μm), both fitted by linear combinations of spectra of carbon cluster families (black solid lines; see Sect. 4 and Table C.2). The separated contributions of flakes and cage populations to the combined spectrum are also shown. In panel a the position of the maximum was adjusted to the astronomical data by shifting the result of the linear combination by +0.17 eV, which falls within the error margin of the present TD-DFTB method.