We have described a program to calculate atomic level population ratios. The code may find a wide range of applicability in astronomical problems, as we have illustrated in the testcases.
We would like to thank F. Haardt for providing us an electronic version of his UVB field. A.I.S. acknowledges finantial support by the Brazilian agency FAPESP, under contract No. 99/05203-8. This work is partially supported by CNPq (304077/77-1) and PRONEX/FINEP (41.96.0908.00).