Describing Anions by Density Functional Theory: Fractional Electron Affinity.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Materials Science

Background:

• Kohn-Sham density functional theory (KS-DFT) is widely used for electronic structure calculations.
• Commonly employed exchange-correlation potentials often exhibit limitations in accurately describing electron behavior, particularly for anions.
• Flexible basis sets can exacerbate issues with electron localization and delocalization.

Purpose of the Study:

• To investigate the accuracy of commonly used exchange-correlation potentials in describing anions within KS-DFT.
• To identify the root cause of incorrect electronic structure descriptions for systems with anionic sites.
• To evaluate the impact of these inaccuracies on systems with both cationic and anionic sites.

Main Methods:

• Utilized Kohn-Sham density functional theory with various exchange-correlation potentials.
• Employed sufficiently flexible basis sets to probe electron behavior at large distances.
• Analyzed systems with both cationic and anionic sites to assess electron transfer phenomena.

Main Results:

• Most tested exchange-correlation potentials describe anions as only partially bound, with excess electrons delocalizing.
• Fractional electron transfer occurs in systems with both positive and negative ions, leading to incorrect electronic structures.
• Exchange functionals lacking Hartree-Fock exchange show pronounced errors, but hybrid functionals are also affected, except for stable anions.

Conclusions:

• The long-range behavior of exchange potentials is identified as the source of the described inaccuracies.
• The long-range correction (LC) method can effectively mitigate these errors.
• These findings have significant implications for the reliability of KS-DFT in describing systems with localized anions or electron-donating sites.