  The properties of dynamical Dark Energy (DE) and, in particular, the
possibility that it can form or contribute to stable inhomogeneities, have been
widely debated in recent literature, also in association to a possible coupling
between DE and Dark Matter (DM). In order to clarify this issue, in this paper
we present a general framework for the study of the nonlinear phases of
structure formation, showing the equivalence between two possible descriptions
of DE: a scalar field \phi self-interacting through a potential V(\phi) and a
perfect fluid with an assigned negative equation of state w(a). This enables us
to show that, in the presence of coupling, the mass of DE quanta may increase
where large DM condensations are present, so that also DE may partake to the
clustering process.
