Developing Orbital-Dependent Corrections for the Non-Additive Kinetic Energy in Subsystem Density Functional Theory

We present a novel route to constructing cost-efficient semi-empirical approximations for the non-additive kinetic energy in subsystem density functional theory. The developed methodology is based on the use of Slater determinants composed of non-orthogonal Kohn$\unicode{x2013}$Sham-like orbitals for the evaluation of kinetic energy expectation values and the expansion of the inverse molecular-orbital overlap matrix into a Neumann series. Applying these techniques, we derived and implemented a series of orbital-dependent approximations for the non-additive kinetic energy, which are employed self-consistently. Our proof-of-principle computations demonstrated quantitatively correct results for potential energy curves and electron densities and hinted on the applicability of the introduced empirical parameters to different types of molecular systems and intermolecular interactions. We therefore conclude that the presented study is an important step towards constructing accurate and efficient orbital-dependent approximations for the non-additive kinetic energy applicable to large molecular systems.