  We present a non-decomposable approximation for the non-additive
non-interacting kinetic energy (NAKE) for covalent bonds based on the exact
behavior of the von Weizs\"{a}cker (vW) functional in regions dominated by one
orbital. This covalent approximation (CA) seamlessly combines the vW and the
Thomas-Fermi (TF) functional with a switching function of the fragment
densities constructed to satisfy exact constraints. It also makes use of
ensembles and fractionally-occupied spin-orbitals to yield highly accurate NAKE
for stretched bonds while outperforming other standard NAKE approximations near
equilibrium bond lengths. We tested the CA within Partition-Density Functional
Theory (P-DFT) and demonstrated its potential to enable fast and accurate P-DFT
calculations.
