Pragmatic Improvement of Magnetic Exchange Couplings from Subsystem Density-Functional Theory through Orthogonalization of Subsystem Orbitals

We explore a pragmatic
way of correcting magnetic exchange coupling
constants from subsystem density-functional theory (sDFT). Our previous
work [Faraday Discuss. 2020, 224, 201−226] showed that sDFT yields robust spin densities
for broken-symmetry-like states, but severely underestimates the magnitude
of exchange couplings when using approximate, density-dependent nonadditive
kinetic-energy functionals. Evaluating the nonadditive kinetic energy
nonselfconsistently by means of potential reconstruction and the exact
single-particle kinetic-energy expression improved the results tremendously,
but this rendered the approach computationally unattractive. Here,
we follow the idea of evaluating the total-system kinetic energy (as
needed for the nonadditive kinetic energy) from a set of approximate
supersystem orbitals, which are simply obtained from orthogonalizing
the sDFT subsystem orbitals. We demonstrate that this leads to a significant
correction of the exchange-coupling constants at low computational
cost. Moreover, the coupling constants calculated in this way are
more robust to a change of the exchange–correlation functional
than the corresponding broken-symmetry (Kohn–Sham-) DFT values.
However, we also identify a significant basis-set dependence in the
results from orthogonalized sDFT.