Two-Orbital Approximation for Partition Density Functional Theory*

Partition Density Functional Theory (P-DFT) is an embedding method in which molecular properties are computed through calculations on the molecule’s constituents. The lack of an accurate approximation for the non-interacting kinetic energy functional in terms of the set of fragment densities makes practical P-DFT calculations a challenge.  We have shown that a simple expression for the non-additive kinetic energy based on a “two-orbital approximation” (2OA)  reproduces extremely well the large-R asymptotic behavior of the dissociation energy of rare-gas dimers with internuclear separation R.  We discuss here the physical motivation behind the construction of the 2OA, its limitations, and how we are extending it to make it more applicable to a broader class of molecules.