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 database of almost 300 carbon–carbon barriers, including 12 poorly behaved that stem from the Y═C—X group, where Y is O or S and X halide. Functionals enhanced exchange mixing 50%) worked well for all barriers. poor actors delocalization errors caused hyperconjugation. These problematic calculations density-sensitive (i.e., DFT predictions change noticeably density), using HF densities (HF-DFT) fixes these issues. For example, conventional B3LYP performs as accurately exchange-enhanced functionals if density used. long-chain conjugated molecules, HF-DFT can be better than functionals. suggest HF-PBE0 has best overall performance.