With the advent of the present and future spatial X-ray missions, it becomes crucial to model correctly the line spectrum of X-ray emitting media. We have built a photoionization code, Titan, solving the transfer of a thousand lines and of the continuum with the Accelerated Lambda Iteration method, which is most reliable for line transfer. In all other photoionization codes the line intensities are computed with the so-called escape probability formalism, used in its simplest approximation. In a previous paper (Dumont et al. 2003), it was shown that this approximation leads to errors which can exceed one order of magnitude in the case a Thomson thickness of the order of unity. We show here that it also happens in the case of a Thomson thickness of 0.001 to 0.1. The errors on the line fluxes and line ratios are of the order of 30% for a column density of 10^{20} cm^{-2}, and a factor five for a column density of 10^{23} cm^{-2}, in conditions appropriate for Seyfert 2 and for the Warm Absorber of Seyfert 1.
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