The most accurate electronic structure methods based on N-particle wave functions are too expensive to be applied to large systems. It is clearer every day the need for treatments of electron correlation that scale favorably with the number of electrons. Among them, the Kohn-Sham formulation of the Density Functional Theory (DFT) has become very popular thanks to its relatively low computational cost. However, present-day functionals have several problems due mainly to the so-called “correlation kinetic energy”, but most importantly, currently available functionals are not N-representable. (more…)