Opacity errors in model atmospheres

Different issues involved in the numerical evaluation of continuous absorption by hydrogenlike atoms and ions have been analyzed. Usually, the calculation of the opacity coefficient is explicitly performed for a predetermined number of levels and the higher ones are treated as an integral; it is shown that there are some cases where as many as 17 discrete levels need to be added before switching to the integral approximation if an error smaller than 0.1% is wanted in the results (a condition not always fulfilled with model atmosphere codes being used). Also, approximations to Gaunt factors are analyzed. Though in some cases approximations can be used which are very good, there are other occasions when it should be advisable to switch to interpolating in original tables, or to use more elaborated expressions (such as Hummer's for the Gaunt free-free factor). A numerical procedure for solving the non linear Pg-Pe-T relation is described in an Appendix.