No need for external orthogonality in subsystem density-functional theory.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Density-Functional Theory

Background:

• Recent studies emphasize the need for externally orthogonal orbitals in subsystem density-functional theory (SDFT).
• This requirement is crucial for accurately representing electron densities in complex molecular systems.

Purpose of the Study:

• To re-investigate the necessity of externally orthogonal orbitals in subsystem density-functional theory (SDFT).
• To determine if supermolecular Kohn-Sham-DFT (KS-DFT) densities can be accurately represented without this orthogonality constraint.

Main Methods:

• Analytical demonstration of density summation.
• Basis-set-free numerical calculations on atomic systems.
• SDFT calculations with reconstructed potentials using finite basis sets.

Main Results:

• Supermolecular KS-DFT densities can be exactly represented as a sum of subsystem densities in the basis-set limit, irrespective of orbital orthogonality.
• SDFT calculations with accurate potentials approach supermolecular KS-DFT density with finite basis sets.
• Deviations between SDFT and KS-DFT diminish as the basis-set limit is approached.

Conclusions:

• External orthogonality is not formally required in SDFT for accurate density representation.
• While not strictly necessary, external orthogonality may still be a useful strategy for developing specific embedding schemes.