A comparison among nine computer codes shows surprisingly large differences where it had been believed that the computational physics was well understood. The codes simulate a plasma that is in steady state but not in local thermodynamic equilibrium (NLTE codes). In this study each code treats an “easy” problem, which is an argon plasma, optically thin and with no external photon flux; densities are varied from near-coronal to an intermediate 1021 electrons/cc and above. The temperatures are high enough that most ions have two or fewer bound electrons, which for this plasma means temperatures above about 300 eV.

The present study asks only if the codes compute similar charge-state populations (a surprising “no”), and, if not, why not. It does not claim accuracy for any code either by comparison to experiment or by appeal to a concensus.