Can density functional theory ‘Cope’ with highly fluxional shapeshifting molecules?

Shape-shifting molecules such as bullvalene undergo rapid structural reorganizations via degenerate Cope rearrangements. Here, we obtain accurate CCSD(T)/CBS barrier heights and reaction energies for a wide range of rearrangements in substituted bullvalenes (C10H9R, R = NH3, OH, CH3, H, F, Cl, SH, CN). We use this benchmark dataset to evaluate the performance DFT ab initio methods kinetics thermodynamics these reactions. The pose significant challenge – best attain root-mean-square deviations 4.9 (BMK), 4.5 (PBE0), 4.2 (PW6B95), 3.8 (B1B95) kJ mol−1. Overall, only three functionals (BMK, PW6B95, MN12-SX) are able surpass (or near) chemical accuracy both energies. In contrast, double-hybrid procedures ωB97X-2(LP), ωB97X-2(TQZ), PWPB95-D3, PBEQI-DH, DSD-PBEB95-D3 give good-to-excellent performance.