Communication: Avoiding unbound anions in density functional calculations.

Area of Science:

• Computational chemistry
• Quantum chemistry
• Electronic structure theory

Background:

• Approximate density functional theory (DFT) methods often fail to accurately bind anions due to significant self-interaction error.
• Hartree-Fock (HF) calculations inherently avoid this self-interaction error, yielding negative highest occupied molecular orbital (HOMO) energies.

Purpose of the Study:

• To evaluate a novel scheme for calculating electron affinities (EAs) by combining DFT approximations with HF densities.
• To demonstrate the effectiveness of this scheme compared to traditional methods, particularly basis set restriction.

Main Methods:

• Electron affinities were computed using total energy differences from DFT approximations (PBE, B3LYP).
• These DFT calculations were performed using electron densities derived from Hartree-Fock (HF) calculations for both neutral and anionic species.
• The accuracy of this hybrid approach was compared against standard DFT methods and basis set restriction techniques.

Main Results:

• The proposed scheme, utilizing HF densities within DFT frameworks, demonstrated high accuracy for calculating electron affinities in molecules.
• This method outperformed the common practice of basis set restriction for most molecular systems.
• Exceptions were noted, such as the cyanide radical (CN), where HF density inaccuracies due to spin contamination affected the results.

Conclusions:

• Combining HF densities with approximate DFT functionals provides a robust and accurate method for calculating molecular electron affinities.
• This approach effectively mitigates the self-interaction error inherent in standard DFT calculations for anions.
• The scheme offers a superior alternative to basis set restriction, except in specific cases with significant spin contamination in the HF density.