Frozen Density Embedding with External Orthogonality in Delocalized Covalent Systems

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Materials Science

Background:

• Frozen density embedding (FDE) is a subsystem DFT method suitable for weakly interacting systems.
• Approximate kinetic energy density functionals (KEDFs) limit FDE's accuracy for covalent and strongly interacting systems.
• External orthogonality (EO) between subsystems can potentially eliminate the need for approximate KEDFs.

Purpose of the Study:

• Implement and generalize external orthogonality (EO) within the FDE framework.
• Overcome limitations of FDE for strongly interacting and covalent systems.
• Accurately reproduce Kohn-Sham DFT energies and densities using subsystem methods.

Main Methods:

• Implementation of EO using the level-shift projection operator method in the Amsterdam density functional program package.
• Generalization of the EO method to remove the need for orbital localization schemes.
• Application to multiple subsystems, including charge-delocalized systems like benzene.

Main Results:

• The implemented EO method successfully reproduces exact Kohn-Sham DFT energies and densities.
• Iterative freeze-and-thaw cycles with EO enable accurate calculations for systems with strong interactions.
• Basis set requirements for subsystems were examined, highlighting the need for bases that describe delocalized orbitals.

Conclusions:

• External orthogonality is a viable approach to enhance the accuracy of frozen density embedding for a wider range of chemical systems.
• The developed method overcomes the limitations of approximate KEDFs in subsystem DFT.
• Accurate description of delocalized orbitals within subsystems is crucial for charge-delocalized systems.