  In this paper we investigate the impact of the coupling between a
quintessence field and clustered matter on the average equation of state of the
scalar field. We take the NFW profile to be characteristic of bound structures
on galactic and cluster scales, and the isothermal distribution to hold for
objects on supercluster scales. Solving analytically for the scalar-field
profile, we find that the greatest impact on the equation of state comes from
the superclusters. Employing numerical case studies, we verify this effect and
probe its dependence on the evolutionary state of the supercluster. An estimate
across the Hubble volume yields corrections to the homogeneous equation of
state of ~3%, increasing with coupling strength.
