Accurate Ionization Energies for Mononuclear Copper Complexes Remain a Challenge for Density Functional Theory

Area of Science:

• Computational Chemistry
• Inorganic Chemistry
• Catalysis

Background:

• Copper's one-electron redox chemistry is vital for many catalytic processes.
• Accurate prediction of ionization energies (IEs) for copper compounds is essential for modeling these reactions.
• Electronic structure methods are key to predicting IEs for copper species with varying oxidation states and ligands.

Purpose of the Study:

• To evaluate the performance of 21 modern density functionals in predicting IEs for 12 mononuclear copper species.
• To compare density functional theory (DFT) results with accurate reference values from the DLPNO-CCSD(T) wave function theory model.
• To identify the most reliable DFT functionals for modeling copper's redox chemistry.

Main Methods:

• Calculated IEs for 12 mononuclear copper species using 21 density functionals.
• Employed the DLPNO-CCSD(T) wave function theory model to establish reference values.
• Included relativistic effects using the zero-order regular approximation for further analysis.

Main Results:

• Significant variation in functional performance was observed across the tested copper species.
• The SOGGA11-X functional demonstrated the best overall performance with a mean unsigned error (MUE) of 0.22 eV.
• PBE0, ωB97X-D, and CAM-B3LYP showed high accuracy, especially when relativistic effects were considered.

Conclusions:

• No single density functional accurately predicts IEs for all tested copper species.
• SOGGA11-X, PBE0, ωB97X-D, and CAM-B3LYP are recommended for modeling copper's redox chemistry.
• Accurate IE prediction is achievable with careful functional selection and consideration of relativistic effects.