  New X-ray observatories (Chandra and XMM-Newton) are providing a wealth of
high-resolution X-ray spectra in which hydrogen- and helium-like ions are
usually strong features. We present results from a new collisional-radiative
plasma code, the Astrophysical Plasma Emission Code (APEC), which uses atomic
data in the companion Astrophysical Plasma Emission Database (APED) to
calculate spectral models for hot plasmas. APED contains the requisite atomic
data such as collisional and radiative rates, recombination cross sections,
dielectronic recombination rates, and satellite line wavelengths. We compare
the APEC results to other plasma codes for hydrogen- and helium-like
diagnostics, and test the sensitivity of our results to the number of levels
included in the models. We find that dielectronic recombination with
hydrogen-like ions into high (n=6-10) principal quantum numbers affects some
helium-like line ratios from low-lying (n=2) transitions.
