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.