Ab Initio Thermochemistry with High-Level Isodesmic Corrections: Validation of the ATOMIC Protocol for a Large Set of Compounds with First-Row Atoms (H, C, N, O, F)

Abstract

The recently proposed ATOMIC protocol is a fully ab initio thermochemical approach designed to provide accurate atomization energies for molecules with well-defined valence structures. It makes consistent use of the concept of bond-separation reactions to supply high-level precomputed bond increments which correct for errors of lower-level models. The present work extends the approach to the calculation of standard heats of formation and validates it by comparison to experimental and benchmark level ab initio data reported in the literature. Standard heats of formation (298 K) have been compiled for a large sample of 173 neutral molecules containing hydrogen and first-row atoms (C, N, O, F), resorting to several previous compilations and to the original experimental literature. Statistical evaluation shows that the simplest implementation of the ATOMIC protocol (composite model C) achieves an accuracy comparable to the popular Gaussian-3 approach and that composite models A and B perform better. Chemical accuracy (1-2 kcal/mol) is normally achieved even for larger systems with about 10 non-hydrogen atoms and for systems with charge-separated valence structures, bearing testimony to the robustness of the bond-separation reaction model. Effects of conformational averaging have been examined in detail for the series of n-alkanes, and our most refined composite model A reproduces experimental heats of formation quantitatively, provided that conformational averaging is properly accounted for. Several cases of larger discrepancy with respect to experiment are discussed, and potential weaknesses of the approach are identified.

Ab initio thermochemistry using optimal-balance models with isodesmic corrections: The ATOMIC protocol

“…The consistent application of BSRs as an integral part of the protocol eliminates the need for dauntingly expensive calculations normally needed if no calibration is permitted and makes the protocol efficient enough to treat systems with about 10-20 nonhydrogen atoms on a standard basis. 71 So far we have only considered molecules composed of hydrogen and first-row ͑C, N, O, F͒ atoms. The ATOMIC approach is not limited to these elements, however, and should in principle be applicable to all molecules for which reasonable valence structures can be drawn, provided that the corresponding BSR prototypes are well-defined stable species.…”

“…The recently introduced G4 method represents a significant improvement over G3 at the expense of somewhat more costly -0. calculations and the introduction of two additional parameters. 35 It seems to be superior to the ATOMIC protocol as well, but Table IV also shows that much of the difference is in fact due to the difficult case of ozone ͑about 4 kcal/mol error for models A and B͒ which G4 appears to get right through calibration ͑0.1 kcal/mol error 35 ͒ and to the missing treatment of conformational averaging for linear alkane chains ͑consistently too negative ⌬H f 0 for models A-C͒, 71 which in G4 appears to be partially compensated for through the parametrization ͑slightly too unstable short chains, pentane: +0.21 kcal/ mol; and only slightly too stable long chains, octane: Ϫ0.30 kcal/mol͒. 35 BSR-corrected model B shows a MUE which is about twice as large as for atomization energies at the CCSD͑T͒ level.…”

“…Full details of the protocol and results for a large set of first-row compounds will be given elsewhere. 71 Here we focus on a statistical evaluation for the subset of neutral, closed-shell HCNOF molecules of the G3/99 standard test set, 131 excluding molecules with two nonhydrogen atoms or less and discarding also tetrafluoroethylene 132,133 and carbonyl fluoride 134 where the results of high-level ab initio benchmark studies have cast some doubt on the accuracy of experimental data.…”

“…Comparison with experiment requires the additional evaluation of zero-point and thermal energies. While an extensive validation with experimental enthalpies of formation will be published elsewhere, 71 Sec. VII already provides a preliminary statistical assessment.…”

Generalized Prediction of Enthalpies of Formation Using DLPNO‐CCSD(T) Ab Initio Calculations for Molecules Containing the Elements H, C, N, O, F, S, Cl, Br

“…A third method is the use of regression coefficients, which is a modification of the atomization approach. Further, proposed calculation methods used with high‐accuracy computational schemes are HEAT, NEAT, the “method of Fishtik,” or ATOMIC …”