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ATOMIC is a thermochemistry protocol geared toward larger molecules with first-row atoms. It implements Pople’s concept of bond separation reactions in an ab initiofashion and so enhances the accuracy of midlevel composite models for atomization energies. Recently we have introduced ATOMIC(hc), a model for applications to hydrocarbons, that estimates bias and uncertainty for each of the components contributing to the ATOMIC bottom-of-the-well atomization energy (Bakowies, D.J. Chem. Theory Comput.2019, 15, 5230−5251). Here we scrutinize the remaining components of the ATOMIC protocol, including midlevel composite models to approximate the complete-basis set (CBS) limit of CCSD(T) as well as zero-point energies (ZPEs) and thermal enthalpy increments that are evaluated from scaled harmonic MP2 frequencies. Potential errors relating to imperfections in MP2 geometries and ZPEs are estimated using auxiliary information obtained from geometry optimizations and frequency calculations at the density functional (B3LYP) level. Overall corrections to and uncertainties of enthalpies of formation are obtained from summation and error propagation, respectively. The error and uncertainty model is validated with accurate data from the Active Thermochemical Tables (ATcT) and compared to earlier statistical assessments for the G3/99 benchmark. The proposed model is a welcome alternative to statistical assessment, first because it does not depend on comparison with experiment, second because it recognizes the expected scaling of error with system size, and third because it provides a detailed account of the importance of various contributions to overall error and uncertainty. The evaluation of ZPEs from scaled harmonic frequencies expectedly emerges as the leading source of uncertainty if highly accurate composite models are used to treat the electronic problem, but uncertainties are usually balanced with those arising from computationally more attractive B level (B1...B6) models to estimate the CBS limit of CCSD(T). ATOMIC(hc) enthalpies of formation, complete with uncertainty estimates, are reported for 161 hydrocarbons ranging in size from methane (CH4) to [8]circulene (C32H16) and tetra-tert-butyltetrahedrane (C20H36). Experimental data are available for 127 molecules but cannot be reconciled with theory in 37 cases. Theory helps to identify the more accurate among conflicting experimental values in 11 cases and emerges as a valuable complement to experiment also for larger molecules, provided that fair estimates of uncertainty are available.

Research in ab initioquantum chemistry has produced an increasing number of thermochemistry protocols, serving different needs from benchmark-level accuracy for small molecules to “chemical accuracy” for larger molecules. While in experimental thermochemistry it is accepted standard to report results complete with intervals of 95% confidence, so far few of the most advanced theoretical approaches have followed suit, based on either statistical comparison to well-established experimental data or careful assessment of high level theoretical results for individual molecules. Here we report on the development of intrinsic uncertainty estimates for the ATOMIC protocol in applications to hydrocarbons. ATOMIC is a theoretical procedure geared toward larger molecules and based on the ab initioimplementation of bond separation reactions (BSRs) to reduce errors of midlevel composite approaches. Each of the components contributing to the bottom-of-the-well atomization energy (EA,e) is scrutinized for possible error by comparison to a large number of very high-level results, including complete-basis-set estimates of CCSDT(Q) bond separation energies for 83 hydrocarbons up to the size of naphthalene. Some of the observations are the following: Post-CCSD(T) effects are sizable even for saturated aliphatic compounds but well-represented in a BSR model summing over bond contributions, while conjugated systems pose more problems. Another significant source of error is the complete-basis-set extrapolation of all-electron CCSD(T) contributions, which still carries an uncertainty of a few tenths of a kcal/mol for midsize molecules like benzene, even if based on large basis-set calculations (cc-pCV5Z, cc-pCV6Z). Scalar relativistic terms and diagonal Born–Oppenheimer corrections are of less concern, the former because they are well represented in a BSR model and the latter because they are small in general. Observations are cast into simple expressions that separate obvious bias from nonsystematic error, formulating the former as correction to and the latter as uncertainty of an ATOMIC result. The updated protocol, complete with uncertainties and termed ATOMIC(hc) (“hc” for hydrocarbons), is validated in comparisons with both experimental data from the Active Thermochemical Tables and high-level theoretical data generated in this work. Analysis of lower-level ATOMIC models and of further components needed to convert EA,einto enthalpies of formation will be reported separately.