Accurate Reference Data for the Nonadditive, Noninteracting Kinetic Energy in Covalent Bonds

The nonadditive, noninteracting kinetic energy (NAKE) is calculated numerically for fragments of H₂, Li₂, Be₂, C₂, N₂, F₂, and Na₂ within partition density functional theory (PDFT). The resulting fragments are uniquely determined, and their sum reproduces the Kohn-Sham molecular density of the corresponding XC functional. We present the NAKE of these unique fragments as a function of internuclear separation and compare the use of fractional orbital occupation to the usual PDFT ensemble method for treating the fragment energies and densities. We also compare Thomas-Fermi and von Weizsäcker approximate kinetic energy functionals to the numerically exact solutions and find significant regions where the von Weizsäcker functional is nearly exact. In addition, we find that the von Weizsäcker approximation can provide accurate NAKE in stretched covalent bonds, especially in the cases of Li₂ and Na₂.