  The observational effect of all minimally coupled scalar field models of dark
energy can be determined by the behavior of the following two parameters : (1)
equation of state parameter $w$, which relates dark energy pressure to its
energy density, and (2) effective speed of sound $c_{e}^{2}$, which relates
dark energy pressure fluctuation to its density fluctuation. In this paper we
show that these two parameters do not uniquely determine the form of a scalar
field dark energy Lagrangian even after taking into account the perturbation in
the scalar field. We present this result by showing that two different forms of
scalar field Lagrangian can lead to the same values for these paired
parameters. It is well known that from the background evolution the Lagrangian
of the scalar field dark energy cannot be uniquely determined. The two models
of dark energy presented in this paper are indistinguishable from the evolution
of background as well as from the evolution of perturbations from a FRW metric.
