Issues on DFT+U calculations of organic diradicals

Area of Science:

• Computational Chemistry
• Materials Science
• Quantum Chemistry

Background:

• Diradical states are crucial for understanding molecular functions and designing functional molecules for molecular devices.
• Density functional theory with plane-wave basis and on-site Coulomb parameter U correction (DFT+U/plane-wave) is a promising method for high-throughput calculations of diradical-band interactions.
• The accuracy of DFT+U/plane-wave calculations for organic diradicals requires detailed investigation.

Purpose of the Study:

• To investigate the accuracy of DFT+U/plane-wave calculations for organic diradicals.
• To identify and analyze the discrepancy in optimum U values between open-shell singlet and triplet states in diradicals.
• To understand the influence of diradical character on the accuracy of DFT+U calculations.

Main Methods:

• Utilized DFT+U/plane-wave calculations with bisphenalenyl molecules as model systems.
• Analyzed the discrepancy in optimum U values between open-shell singlet and triplet electronic states.
• Compared DFT+U results with hybrid-DFT calculations and discussed reference values like local magnetic moments and unpaired electron numbers.

Main Results:

• Detected a discrepancy in optimum U values between singlet and triplet states of organic diradicals.
• Attributed this discrepancy to differences in electronic delocalization via π-conjugation.
• Found that the effect of U discrepancy increases as the diradical character decreases, and DFT+U underestimates the HOMO-LUMO gap.

Conclusions:

• Accurate calculation of organic diradicals, especially those with low diradical character, necessitates careful consideration of U value discrepancies, potentially referencing more accurate methods or experimental data.
• While effective magnetic exchange integral values show partial cancellation of U discrepancy effects, caution is advised.
• DFT+U calculations provide a qualitative understanding of the HOMO-LUMO gap but underestimate its magnitude for bisphenalenyls.