  This paper is concerned with theoretical estimates of the refractive--index
curves for quartz, obtained by the Kubo formul\ae\ in the classical
approximation, through MD simulations for the motions of the ions. Two
objectives are considered. The first one is to understand the role of
nonlinearities in situations where they are very large, as at the
$\alpha$--$\beta$ structural phase transition. We show that on the one hand
they don't play an essential role in connection with the form of the spectra in
the infrared. On the other hand they play an essential role in introducing a
chaoticity which involves a definite normal mode. This might explain why that
mode is Raman active in the $\alpha$ phase, but not in the $\beta$ phase. The
second objective concerns whether it is possible in a microscopic model to
obtain normal mode frequencies, or peak frequencies in the optical spectra,
that are in good agreement with the experimental data for quartz.
Notwithstanding a lot of effort, we were unable to find results agreeing better
than about 6%, as apparently also occurs in the whole available literature. We
interpret this fact as indicating that some essential qualitative feature is
lacking in all models which consider, as the present one, only short--range
repulsive potentials and unretarded long--range electric forces.
