Approximate
exchange–correlation functionals built by modeling
in a nonlinear way the adiabatic connection (AC) integrand of density
functional theory have many attractive features, being virtually parameter-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 second-order perturbation theory.