  We implemented non-solar-scaled opacities in a simultaneous derivation of
fundamental parameters and abundances. We carried out a high-precision stellar
parameters and abundance determination by applying non-solar-scaled opacities
and model atmospheres. Our sample is composed by 20 stars including
main-sequence and evolved objects. Opacities for an arbitrary composition and
vmicro were calculated through the Opacity Sampling method. We obtained a small
difference in stellar parameters derived with non-solar-scaled opacities
compared to classical solar-scaled models. The differences in Teff, log g and
[Fe/H] amount up to 26 K, 0.05 dex and 0.020 dex for the stars in our sample.
These differences could be considered as the first estimation of the error due
to the use of classical solar-scaled opacities to derive stellar parameters
with solar-type and evolved stars. Some chemical species could also show an
individual variation higher than those of the [Fe/H] (up to 0.03 dex) and
varying from one specie to another, obtaining a chemical pattern difference
between both methods. This means that condensation temperature Tc trends could
also present a variation. We include an example showing that using
non-solar-scaled opacities, the classical solution cannot always verify the
excitation and ionization balance conditions required for a model atmosphere.
We consider that the use of the non-solar-scaled opacities is not mandatory in
every statistical study with large samples of stars. However, for those
high-precision works whose results depend on the mutual comparison of different
chemical species (such as Tc trends), we consider that it is worthwhile its
application. To date, this is probably one of the more precise spectroscopic
methods of stellar parameters derivation [abridged].
