Calculated and observed optical spectrum for ζ Puppis (lower three panels; the model spectrum has been convolved with a rotational broadening profile and an instrumental gaussian). Although the spectrum in this wavelength range is also computed by our code, it is not our primary means of spectral diagnostics, because compared to the UV spectral range (upper three panels) it is obviously just “the tail of the beast”, since it contains only few and only rather weak lines. When compared on the same vertical scale (as shown here) it becomes immediately obvious that the UV spectral range contains much more information, and due to the much stronger lines in the UV part of the spectrum, individual features are far less sensitive to minor uncertainties in the modelling than in the optical range.