Multi-epoch spectra of SN 1998bw (centre; corrected for redshift and extinction), and of models r6e4BH (top) and r0e2 (bottom). Times are given since MJD 50 929.4 for SN 1998bw and since explosion for the models. We shade the model flux associated with bound-bound transitions of specific ions (although instructive, this procedure only works accurately in the absence of line overlap). Neither model matches the whole evolution of SN 1998bw. However, the lower mass and higher energy model r6e4BH more closely resembles SN 1998bw at early times while the more massive and lower energy model r0e2 matches better at late times.