Contribution function G_λ0(T,n_e) for Fe ix 171 and Fe xii 193. The maximum formation temperature for these ions are, respectively, log T = 5.9 and log T = 6.2 K. The black parallelogram in each surface has its corners defined by the minimum and maximum temperature and density values found in the base model, and therefore includes the G_λ0(T,n_e) values used here. The temperature range in the upper panel has been inverted in order to show the parallelogram for the G₁₇₁(T,n_e) surface.