Constructing fragment-density functionals

Approximations of the non-additive non-interacting kinetic energy (NAKE) as an explicit functional of the density are the basis of fragment-based methods that provide improved computational efficiency over standard Kohn-Sham calculations. However, within most fragment-based formalisms, there is no unique NAKE, making it difficult to develop general, robust approximations to it. In Partition Density Functional Theory, the ambiguity of the NAKE is removed and approximate functionals may be more meaningfully compared to exact quantities. We discuss promising avenues for constructing fragment density-functionals for the NAKE: (1) By re-parametrizing decomposable conjoint functionals for weakly-bonded systems; (2) By designing non-decomposable functionals for weak and covalent bonds that satisfy exact constraints of the partition energy without fitting.