On the calculation of atomic term populations

The usefulness of calculations on model atomic term systems which can give spectral multiplet intensities is emphasized, in contrast to more detailed level calculations which are not always feasible because of lack of appropriate atomic data. A more general expression for the multiplet radiative transition rate is proposed to facilitate term representations. The differences between term and level representations are discussed quantitatively with respect to a model three-level atom and real examples of the C III and Ne IV ions. It is shown that term representations fail at lower densities when level inverse lifetimes within terms differ by only a few orders of magnitude. In such cases one must resort to other methods; a hybrid calculation is therefore proposed to fill this need and is carried out for the C III ion to demonstrate its feasibility and validity.