  The building of a stellar structure requires knowing the Rosseland mean
opacity at each layer of the model. This mean opacity is very often
interpolated in pre-computed tables due to the overwhelming time to compute it
from monochromatic cross sections. The main drawback to using tables is that
the opacities can be inconsistent with the actual local chemical composition,
for instance in the regions of the star where nucleosynthesis occurs. We
developed a strategy that allows very fast calculations of Rosseland opacities
from monochromatic cross sections. This method has been implemented in the
Toulouse-Geneva evolution code, which we used to compute evolutionary tracks
with models whose Rosseland opacities are fully consistent with the chemical
mix everywhere in the star. Our self-consistent models show very small
structural differences compared to models where the Rosseland opacity is
computed with a fixed metal mixture. The main-sequence evolutionary tracks are
almost the same for models of mass ranging from 2 to 8 solar masses. At a given
surface gravity the relative difference in age is lower than 2% and generally
below 1% between the two kinds of calculations, the self-consistent model being
younger most of the time. Unless such a precision in age is sought out, the use
of tabulated Rosseland opacities with a metal content defined globally is still
acceptable, at least in main-sequence stars where the chemical mix changes only
through nucleosynthesis.
