  We consider a double polytropic cosmological fluid and demonstrate that, when
one constituent resembles a bare cosmological constant while the other emulates
a generalized Chaplygin gas, a good description of the Universe's large-scale
dynamics is obtained. In particular, our double polytropic reduces to the
Murnaghan equation of state, whose applications are already well established in
solid state physics and classical thermodynamics. Intriguingly, our model
approximates the conventional $\Lambda$CDM paradigm while reproducing the
collective effects of logotropic and generalized Chaplygin fluids across
different regimes. To check the goodness of our fluid description, we analyze
first order density perturbations, refining our model through various orders of
approximation, utilizing $\sigma_8$ data alongside other cosmological data
sets. Encouraging results suggest that our model, based on the Murnaghan
equation of state, outperforms the standard cosmological background within
specific approximate regimes and, on the whole, surpasses the standard
phenomenological reconstruction of dark energy.
