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 (P-DFT), the ambiguity of the NAKE is removed and approximate functionals may be more meaningfully compared to exact quantities. We demonstrate that the decomposable approximations constructed from approximations to the non-interacting kinetic energy fail to reproduce accurate NAKE and 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. We derive two Virial relations for P-DFT which can be useful in constructing functionals for the NAKE.
Wasserman, Purdue University.
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