  Atomic physics calculations of radiative cooling are used to calculate
criteria for the overstability of radiating shocks. Our calculations explain
the measurement of shock overstability by Grun et al. and explain why the
overstability was not observed in other experiments. The methodology described
here can be especially useful in astrophysical situations where the relevant
properties leading to an overstability can be measured spectroscopically, but
the effective adiabatic index is harder to determine.
