  Dynamical models for dark energy are an alternative to the cosmological
constant. It is important to investigate properties of perturbations in these
models and go beyond the smooth FRLW cosmology. This allows us to distinguish
different dark energy models with the same expansion history. For this, one
often needs the potential for a particular expansion history. We study how such
potentials can be reconstructed obtaining closed formulae for potential or
reducing the problem to quadrature. We consider three classes of models here:
tachyons, quintessence, and, interacting dark energy. We present results for
constant w and the CPL parameterization. The method given here can be
generalized to any arbitrary form of w(z).
