Models: The grid \[grids1\] {#models-the-grid-grids1 .unnumbered}
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Here we describe the building of the grid of models. We use the photoionization code [^3] v 2.2 with the most up-to-date atomic database v 2.2.1bn20 ([@bautista2001a]).

The code includes all the relevant atomic processes, including inner shell processes ([@palmeri2003a; @palmeri2003b]). It computes the emissivities and optical depths of the most prominent X-ray and UV lines identified in AGN spectra. Our models are based on spherical slabs illuminated by a point-like X-ray continuum source. The input parameters are the source spectrum, the gas composition, the gas density $n_{H}$, the column density in the line of sight $N_{H}$ and the ionization parameter $\xi$. The source spectrum is described by the spectral luminosity $L_{\epsilon}=L_{\rm ion}f_{\epsilon}$, where $L_{\rm ion}$ is the integrated luminosity from 1 to 1000 Ryd, and $\int_{1}^{\rm 1000~Ryd} f_{\epsilon}
d\epsilon=1$. The spectral function $f_{\epsilon}$ is taken to be a powerlaw $\sim \epsilon^{\alpha}$ with $\alpha=-1$ (photon index $\Gamma_{\rm x}=2$). The gas consists of the following elements, H, He, C, N, O, Ne, Na, Mg, Si, S, Ar, Ca and Fe. We use the abundances of [@grevesse1996a] in all our models (and the term [*solar*]{} for these abundances). We adopt a turbulent velocity of 200 