  We report an update and enhancement of the ACONFL (conformer energies of
large alkanes [Ehlert, S.; Grimme, S.; Hansen, A. J. Phys. Chem. A 2022, 126,
3521-3535]) dataset. For the ACONF12 (n-dodecane) subset, we report basis set
limit canonical CCSD(T) reference data obtained from MP2-F12/cc-pV{T,Q}Z-F12
extrapolation, [CCSD(F12*)-MP2-F12]/aug-cc-pVTZ-F12, and a (T) correction from
conventional CCSD(T)/aug-cc-pV{D,T}Z calculations. Then we explored the
performance of a variety of single and composite localized-orbital CCSD(T)
approximations, ultimately finding an affordable LNO-CCSD(T)-based post-MP2
correction that agrees to 0.008 kcal/mol MAD (mean absolute deviation) with the
revised canonical reference data. In tandem with canonical MP2-F12/CBS
extrapolation, this was then used to re-evaluate the ACONF16 and ACONF20
subsets for n-hexadecane and n-icosane, respectively. A revised ACONFL set was
thus obtained. It was then used to assess the performance of different
localized-orbital coupled cluster approaches, such as PNO-LCCSD(T) as
implemented in MOLPRO, DLPNO-CCSD (T1) as implemented in ORCA, and LNO-CCSD(T)
as implemented in MRCC, at their various accuracy settings. A three-tier
LNO-CCSD(T)-based composite scheme disagrees by only 0.02 kcal/mol from the
revised ACONFL reference data. When extrapolated to the complete PNO space
limit, DLPNO-CCSD(T1, Tight) and a composite method are the best picks among
all the localized coupled cluster methods tested for the dodecane conformers.
Dispersion-corrected dRPA-based double hybrids perform remarkably well for the
ACONFL set. While the revised reference data do not affect any conclusions on
the less accurate methods, they may upend orderings for more accurate methods
with error statistics on the same order as the difference between reference
datasets.
