High-Accuracy Heats of Formation for Alkane Oxidation: From Small to Large via the Automated CBH-ANL Method

It is generally challenging to obtain high-accuracy predictions
for the heat of formation for species with more than a handful of
heavy atoms, such as those of importance in standard combustion mechanisms.
To this end, we construct the CBH-ANL approach and illustrate that,
for a set of 194 alkane oxidation species, it can be used to produce
ΔHf(0 K) values with 2σ uncertainties
of 0.2–0.5 kcal mol–1. This set includes
the alkanes, hydroperoxides, and alkyl, peroxy, and hydroperoxyalkyl
radicals for 17 representative hydrocarbon fuels containing up to
10 heavy atoms with various degrees of branching in the alkane backbone.
The CBH-ANL approach, automated in the QTC and AutoMech software suites,
builds balanced chemical equations for the calculation of ΔHf(0 K), in which the reference species may be
up to five heavy atoms. The high-level ANL0 and ANL1 reference ΔHf(0 K) values are further refined for even the
largest of these reference species with a novel laddering approach.
We perform a comprehensive quantification of the uncertainties for
both the individual reference species (the largest of which is 0.15
kcal mol–1) and the propagation of those uncertainties
when used in the calculation of ΔHf(0 K) for the 194 target species. We examine the sensitivity of the
predicted ΔHf(0 K) values to (i)
electronic energies from various methods, including ωB97X-D/cc-pVTZ,
B2PLYP-D3/cc-pVTZ, CCSD­(T)-F12b/cc-pVDZ-F12//B2PLYP-D3/cc-pVTZ, and
CCSD­(T)-F12b/cc-pVTZ-F12//B2PLYP-D3/cc-pVTZ; (ii) the zero-point vibrational
energies (ZPVEs), where we consider harmonic ZPVEs as well as two
scaling-based estimates of the anharmonic ZPVEs, all implemented for
both ωB97X-D/cc-pVTZ and B2PLYP-D3/cc-pVTZ calculations; (iii)
the particular CBH-ANL scheme employed; and (iv) the procedure for
choosing the reference conformer for the analyses. The discussion
concludes with a summary of the estimated overall uncertainty in the
predictions and a validation of the predictions for the alkane subset.