Well, you have to keep your tounge straight here. We're a very very far way from doing DFT of protein folding, that's correct. But we have little reason to beli

Well, you have to keep your tounge straight here. We're a very very far way from doing DFT of protein folding, that's correct. But we have little reason to believe that DFT is useful for studying the interactions of macromolecules; these interactions are dominated by typical van der Waals forces, along with repulsion and electrostatics, and this doesn't need a quantum mechanical treatment. Then you're in the world of molecular dynamics etc., where we have no problems simulating two enzymes interacting. See this paper where they did fully atomistic simulations of an entire HIV capsule back in 2013: http://www.nature.com/nature/journal/v497/n7451/full/nature12162.html http://www.nature.com/nature/journal/v497/n7451/full/nature1...