Density Functional Theory of molecular fragments: Strongly-correlated electrons in weakly-interacting fragments

To gain flexibility in the search for the minimum energy in ground-state DFT, and to improve on the accuracy of density functional approximations for stretched bonds, we explore a generalized DFT in which the ground-state energy of a molecule is decomposed into a sum of fragment energies and a partition energy. The latter is defined as a functional of the set of fragment spin densities and is amenable to simple, physically-motivated approximations designed to satisfy exact constraints that functionals of the total molecular spin densities cannot easily meet. Employing standard XC approximations for the fragments, this approach is successful at dissociating challenging molecules correctly without artificially breaking their spin symmetries. We discuss implications for strongly-correlated electronic systems, and challenges ahead.