Communication: Density functional theory embedding with the orthogonality constrained basis set expansion procedure

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Density functional theory (DFT) embedding methods are crucial for large system calculations.
• Subsystem treatments at different theory levels require ensuring orbital orthogonality.
• Existing projector methods often involve complex parameters or approximations.

Purpose of the Study:

• To implement and evaluate the orthogonality constrained basis set expansion (OCBSE) procedure for DFT-in-DFT embedding.
• To provide a parameter-free method for enforcing subsystem orbital orthogonality.
• To assess the accuracy and convergence of OCBSE compared to existing methods.

Main Methods:

• Implementation of the orthogonality constrained basis set expansion (OCBSE) procedure.
• Application of OCBSE within a DFT-in-DFT embedding framework.
• Testing on three representative chemical systems.

Main Results:

• OCBSE successfully enforces subsystem orbital orthogonality without level shifting.
• Excellent convergence behavior was achieved without freezing subsystem densities.
• Accuracy comparable to or exceeding the Huzinaga scheme with frozen densities.

Conclusions:

• OCBSE is a simple, parameter-free, and effective alternative for DFT-in-DFT embedding.
• Allowing density response enhances flexibility and applicability.
• OCBSE shows promise for future extensions, including wavefunction-DFT embedding.