  Observations suggest that a relationship exists between the driving mechanism
of roAp star pulsations and the heavy element distribution in these stars. We
attempt to study the effects of local and global metallicity variations on the
excitation mechanism of high order p-modes in A star models. We developed
stellar evolutionary models to describe magnetic A stars with different global
metallicity or local metal accumulation profiles. These models were computed
with CLES ("Code Li\`egeois d'\'evolution stellaire"), and the stability of our
models was assessed with the non-adiabatic oscillation code MAD. Our models
reproduce the blue edge of the roAp star instability strip, but generate a red
edge hotter than the observed one, regardless of metallicity. Surprisingly, we
find that an increase in opacity inside the driving region can produce a lower
amount of driving, which we refer to as the "inverse $\kappa$-mechanism".
