like ion column density ($N_{ion}$) using the strength of absorption lines. However, the technique is full of exact requirements and limitations. In order for a line to be used as COG diagnostic tool, firstly (1) a well defined continuum level has to be known prior any construction of the COG. Secondly (2) the synthetic COGs, are usually compare with real measurements of equivalent widths (EWs) of lines. Third (3) the chosen lines must be not saturated respect to the variable we wish to contrast to. Furthermore (4), a reliable [*identification*]{} procedure [*must be*]{} built on the grounds of a well constructed photoionization modelling ([@kallman2001a]) which in turn needs a well established set of atomic data. For instance, [@badnell2006a] pointed out about the inappropriate use of some dielectronic recombination (DR) rates, which are later discussed by [@kallman2010a] in the context of photoionization modelling and the correct use of atomic data (now implemented in the latest version of ) ([@bautista2001a]).

Following a series of papers related with the computations of K lines of Fe ([@palmeri2002a; @palmeri2003a]), and K-shell photoabsorption of O ions ([@ramirez2002a; @garcia2005a; @garcia2011b] atomic data (energies, cross sections, lifetimes), [@palmeri2008a]) discuss in detail the reliability and accuracy of some sets 