  Accurate treatment of the electronic correlation in inhomogeneous electronic
systems, combined with the ability to capture the correlation energy of the
homogeneous electron gas, allows to reach high predictive power in the
application of density-functional theory. For two-dimensional systems we can
achieve this goal by generalizing our previous approximation [Phys. Rev. B 79,
085316 (2009)] to a parameter-free form, which reproduces the correlation
energy of the homogeneous gas while preserving the ability to deal with
inhomogeneous systems. The resulting functional is shown to be very accurate
for finite systems with an arbitrary number of electrons with respect to
numerically exact reference data.
