  The possibility of unifying dark-matter and dark-energy has recently
attracted considerable interest. In this so called quartessence scenario, a
single component is responsible for both the clustering of matter and the
accelerated expansion of the universe. A model archetype for such scenario is
provided by the Chaplygin gas. Although this model is in agreement with the
data on the expansion history, problems arise in the power spectrum of density
fluctuations for adiabatic perturbations. In this contribution we consider
other quartessence models and confirm that instabilities and oscillations in
the matter power spectrum are a characteristic of more generic quartessence
models, namely those with a convex equation of state. We show that, as in the
Chaplygin case, this kind of problem can be solved by considering intrinsic
non-adiabatic perturbations such that, as an initial condition, the perturbed
fluid is gradient pressure free. We also discuss how the problems of adiabatic
quartessence can be circumvented by other types of equations of state.
