  Approximate exchange-correlation functionals built by modeling in a
non-linear way the adiabatic connection (AC) integrand of density functional
theory have many attractive features, being virtually parameters-free and
satisfying different exact properties, but they also have a fundamental flaw:
they violate the size-consistency condition, crucial to evaluate interaction
energies of molecular systems. We show that size consistency in the AC-based
functionals can be restored in a very simple way at no extra computational
cost. Results on a large set of benchmark molecular interaction energies show
that functionals based on the interaction strength interpolation approximations
are significantly more accurate than the second-order perturbation theory.
