Explaining and Fixing DFT Failures for Torsional Barriers

Most torsional barriers are predicted with high accuracies (about 1 kJ/mol) by standard semilocal functionals, but a small subset was found to have much larger errors. We created a database of almost 300 carbon-carbon torsional barriers, including 12 poorly behaved barriers, that stem from the Y=C-X group, where Y is O or S and X is a halide. Functionals with enhanced exchange mixing (about 50%) worked well for all barriers. We found that poor actors have delocalization errors caused by hyperconjugation. These problematic calculations are density-sensitive (i.e., DFT predictions change noticeably with the density), and using HF densities (HF-DFT) fixes these issues. For example, conventional B3LYP performs as accurately as exchange-enhanced functionals if the HF density is used. For long-chain conjugated molecules, HF-DFT can be much better than exchange-enhanced functionals. We suggest that HF-PBE0 has the best overall performance.

Automated summary

The majority of torsional barriers can be accurately predicted by standard semilocal functionals, but some outliers, particularly in the Y=C-X group, exhibit larger errors due to delocalization errors caused by hyperconjugation. Using HF densities can improve the accuracy of calculations for these problematic cases, with HF-DFT showing better performance for long-chain conjugated molecules compared to exchange-enhanced functionals. HF-PBE0 is suggested to have the best overall performance in this study.