  Conformational energies of n-butane, n-pentane, and n-hexane have been
calculated at the CCSD(T) level and at or near the basis set limit.
Post-CCSD(T) contribution were considered and found to be unimportant. The data
thus obtained were used to assess the performance of a variety of density
functional methods. Double-hybrid functionals like B2GP-PLYP and B2K-PLYP,
especially with a small Grimme-type empirical dispersion correction, are
capable of rendering conformational energies of CCSD(T) quality. These were
then used as a `secondary standard' for a larger sample of alkanes, including
isopentane and the branched hexanes as well as key isomers of heptane and
octane. Popular DFT functionals like B3LYP, B3PW91, BLYP, PBE, and PBE0 tend to
overestimate conformer energies without dispersion correction, while the M06
family severely underestimates GG interaction energies. Grimme-type dispersion
corrections for these overcorrect and lead to qualitatively wrong conformer
orderings. All of these functionals also exhibit deficiencies in the conformer
geometries, particularly the backbone torsion angles. The PW6B95 and, to a
lesser extent, BMK functionals are relatively free of these deficiencies.
  Performance of these methods is further investigated to derive conformer
ensemble corrections to the enthalpy function, $H_{298}-H_0$, and the Gibbs
energy function, ${\rm gef}(T)\equiv - [G(T)-H_0]/T$, of these alkanes. While
$H_{298}-H_0$ is only moderately sensitive to the level of theory, ${\rm
gef}(T)$ exhibits more pronounced sensitivity. Once again, double hybrids
acquit themselves very well.

Chemical Physics;Atomic and Molecular Clusters