Thus, we obtain a new type of density-functional approximation (with an implicit dependence on the density) where we can afford an accurate description of strong electron correlation. Obviously, in general, the cluster's environment (which is usually neglected) contributes to the total correlation energy. A formally exact density-functional embedding theory would in principle be obtained by deriving, in this context, a multi-reference version of Görling-Levy perturbation theory. (4.2.9) Grimme : While Kohn-Sham-DFT calculations with accurate DFAs in combination with good one-particle basis sets are feasible for molecules with a few hundreds of atoms, they are still computationally too demanding for many purposes -for example in large scale screening applications, for the combinatorial problem of conformational sampling of flexible molecules, or the computation of vibrational Gibbs free energies of large systems. While the initial steps in typical multilevel approaches can be conducted routinely at a semiempirical or force-field level, 281 at some point in the applied filtering procedures, higher accuracy is required -in particular, for relative (chemical) energies.