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.