Conformational equilibria in butane-1,4-diol: a benchmark of a prototypical system with strong intramolecular H-bonds

Explicitly correlated CCSD(T) valence basis limit relative energies were obtained for the 65 conformers of butane-1,4-diol, a prototypical system with a strong internal hydrogen bond. The performance of a variety of ab initio and DFT methods (with and without empirical dispersion corrections) was assessed in detail. Consideration of all pairwise conformer energies provides a performance gauge for both H-bonds and van der Waals interactions, aside from internal strain of angles and bonds. In the post-HF realm, it was found that SCS(MI)CCSD-F12/cc-pVDZ-F12 can be a cost-effective alternative to CCSD(T)/CBS, almost without any loss in accuracy. In the DFT arena, the double-hybrid DSD-PBEP86-D3BJ surpasses the accuracy of all other methods (except for SCS(MI)CCSD, but at a small fraction of its cost). Several hybrid functionals provide an acceptable accuracy with the def2-QZVP basis set, especially BMK, M06, LC-ωPBE-D3, and TPSS0-D3. With the more modest 6-311+G(d,p) basis set, the H-bonds are far from basis set completeness and, due to error compensation, the inclusion of a dispersion correction is generally counterproductive. Some functionals that represent "Pauling points" at this level are LC-ωPBE, TPSS0, B1B95, BMK, TPSSh, PBE0, TPSS, and ωB97X.