Upper panel: radiative cooling function derived using emissivities from AtomDB (in the low-density limit, see Sect. 4.2) and abundances (in units of the solar abundance of Anders & Grevesse 1989) of O = 0.92, Ne = 1.31, Ar = 0.93, Ca = 1.07, Fe = 0.61 (see Sect. 4.1). Lower panel: line emissivities as a function of temperature from AtomDB, normalized by their peak emissivity, for the H-like (full, black line) and He-like resonance+intercombination (dotted, red line) transitions of O, Ne, Mg, Si, S, Ar, Ca, and Fe.