Density differences in embedding theory with external orbital orthogonality.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Density Functional Theory

Background:

• Density Functional Theory (DFT) is a powerful tool for electronic structure calculations.
• DFT-in-DFT embedding methods aim to reduce computational cost by treating subsystems separately.
• Traditional DFT-in-DFT methods can suffer from inaccuracies, particularly at subsystem boundaries.

Purpose of the Study:

• To introduce and validate a new DFT-in-DFT embedding methodology.
• To improve the accuracy of electron densities and energies in molecular complexes.
• To develop a method that avoids error-prone potentials and does not require supermolecular calculations.

Main Methods:

• Development of Kohn-Sham equations with constrained electron density (KSCED) and external orbital orthogonality (ext orth).
• Implementation of KSCED(x, ext orth) embedding, where x can be monomer (m), supermolecular (s), or extended monomer (e).
• Calculation of electron densities, energies, and potential energy curves (PECs) for molecular complexes with varying interaction strengths.

Main Results:

• The new DFT-in-DFT approach achieves significantly higher accuracy compared to previous protocols.
• Enforcing external orbital orthogonality remedies errors in electron density at subsystem boundaries.
• KS-DFT total energies are reproduced to high precision, and PECs are virtually identical to conventional KS-DFT results.
• The extended monomer expansion provides results closely related to supermolecular calculations without needing them.

Conclusions:

• The developed DFT-in-DFT embedding equations with external orbital orthogonality offer a highly accurate and computationally efficient alternative.
• This method overcomes limitations of traditional DFT-in-DFT approaches, particularly regarding boundary errors.
• The new approach provides a robust framework for electronic structure calculations of molecular complexes.