In the outermost level of the nested iteration scheme we also iterate for the temperature structure of the atmosphere using a generalization of the Unsöld-Lucy temperature correction scheme to spherical geometry and NLTE model calculations. This has proven to work very well even in extreme NLTE cases such as nova and supernova atmospheres. The temperature correction procedure also requires virtually no memory and CPU time overheads. The Unsöld-Lucy correction scheme (see Mihalas [34] for a discussion of this and other temperature correction schemes), uses the global constraint equation of energy conservation to find corrections to the temperature that will fulfill energy conservation better than the previous temperatures. We have found it to be more stable than a Newton-Raphson linearization scheme and it allows us to separate the temperature corrections from the statistical equations discussed above.

To derive the Unsöld-Lucy correction, one uses the fact that
the *ratios* of the wavelength averaged absorption and
extinction coefficients

Dropping terms of order (*v*/*c*), one can then
use the angular moments of the SSRTE to show that in order to
obtain radiation equilibrium *B* should be corrected by an amount

The first term in Eq. 35 corresponds simply to a iteration term
and will thus provide too small temperature corrections in the *inner*
parts of the atmosphere (but work fine in the outer, optically thin parts). The
second term of Eq. 35, however, is the dominant term in the inner
parts of the atmosphere. It provides a very good approximation to the
temperature corrections deep inside the atmosphere. Following
[35], we found that it is sometimes better to modify this general
scheme by, e.g., excluding the contributions of extremely strong lines to the
opacity averages used in the calculations because they tend to
dominate the average opacity but do not contribute as much to the total error
in the energy conservation constraint.