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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Benchmarking Compound Methods (CBS-QB3, CBS-APNO, G3, G4, W1BD) against the Active Thermochemical Tables: Formation Enthalpies of Radicals
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Benchmarking Compound Methods (CBS-QB3, CBS-APNO, G3, G4, W1BD) against the Active Thermochemical Tables: Formation Enthalpies of Radicals

机译：对照有效热化学表的基准化合物方法（CBS-QB3，CBS-APNO，G3，G4，W1BD）：自由基的形成焓

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摘要

The 298.15 K formation enthalpies of 38 radicals with molecular formula CxHyOz have been computed via the atomization procedure using the five title methods. The computed formation enthalpies are then benchmarked against the values recommended in the Active Thermochemical Tables (ATcT). The accuracy of the methods have been interpreted in terms of descriptive statistics, including the mean-signed error, mean-unsigned error, maximum average deviation, 2 sigma uncertainties, and 2Xroot-mean-square-deviations (2RMSD). The results highlight the following rank order of accuracy for the methods studied G4> G3 > W1BD > CBS-APNO > CBS-QB3. The findings of this work are also considered in light of a recent companion study, which took an identical approach to quantifying the accuracies of these methods for 48 closed-shell singlet CxHyOz compounds. A similar order of accuracies and precisions were observed therein: G3 > G4 > W1BD > CBS-APNO > CBS-QB3. Both studies highlight systematic biases/deviations from the ATcT for the methods investigated, which are discussed in some detail, with methods having clear tendencies to over- or underpredict the recommended formation enthalpies for radical and/or closed-shell CxHyOz compounds. We show that one can improve the accuracy of their computation, and simultaneously reduce the uncertainty, by taking unweighted average formation enthalpies from various combinations of methods used. The reader should note that the statistical analyses preceding these conclusions also highlight that these error cancellation effects are unique for closed-shell and radical species. By extension, these error-cancellation effects can be expected to be different for various homologous series and chemical functionalities and their closed- and open-shell subgroups. Hence, further benchmarking studies are advised for other homologous series, such that the scientists and engineers (e.g., combustion/atmospheric/astrochemical) who frequently use these methods can assign reasonable uncertainties to their computations, while simultaneously optimizing their computational costs. For CxHyOz compounds, a combination of the CBS-APNO/G3/G4 methods is shown to be quite powerful when the atomization method is employed and is capable of reproducing the ATcT to within "near-chemical-accuracy", with 2RMSD (approximate to 95% confidence interval) values of 0.0 +/- 4.34 kJ mol(-1) computed for CxHyOz radicals, 0.0 +/- 4.22 kJ mol(-1) for closed-shell CxHyOz compounds, with a total uncertainty of 0.0 +/- 4.27 kJ mol(-1) subsequently computed considering all 85 CxHyOz compounds. Given the performance of these methods for determination of formation enthalpies when the atomization procedure is employed, we expect isodesmic reactions involving these methods to be capable of achieving chemical accuracy, as illustrated for the case of the tert-butyl radical. We also highlight that there is still disagreement between experiment and theory for this radical, despite its significance in gas-phase chemistry. Kineticists, thermodynamicists, and chemical kinetic modellers alike are warned that the popular CBS-QB3 method is found to have particularly poor performance, with a computed 2RMSD of 0.0 +/- 12.
机译：使用五种标题方法，通过雾化程序计算了分子式为CxHyOz的38个自由基的298.15 K形成焓。然后将计算出的生成焓与活性热化学表（ATcT）中推荐的值进行比较。该方法的准确性已通过描述性统计进行了解释，包括均值误差，均值无符号误差，最大平均偏差，2 sigma不确定性和2X均方根偏差（2RMSD）。结果突出表明，对于所研究的方法，G4> G3> W1BD> CBS-APNO> CBS-QB3的准确性依次为以下等级。还根据最近的一项伴随研究考虑了这项工作的发现，该研究采用了相同的方法来量化这些方法对48种闭壳单峰CxHyOz化合物的准确性。在其中观察到相似的精度和精度顺序：G3> G4> W1BD> CBS-APNO> CBS-QB3。两项研究均突出显示了所研究方法与ATcT的系统性偏差/偏差，对此进行了详细讨论，这些方法具有明显倾向于过高或过低预测CxHyOz自由基和/或闭壳CxHyOz化合物的推荐形成焓的趋势。我们表明，通过从使用的各种方法组合中获取未加权的平均地层焓，可以提高其计算的准确性，同时减少不确定性。读者应注意，在得出这些结论之前进行的统计分析还强调，这些误差消除效应对于闭壳和自由基物种而言是独特的。通过扩展，对于各种同源系列和化学官能团以及它们的闭壳和开壳亚组，可以预期这些错误消除效应是不同的。因此，建议对其他同源序列进行进一步的基准研究，以便经常使用这些方法的科学家和工程师（例如燃烧/大气/天化学）可以为计算分配合理的不确定性，同时优化计算成本。对于CxHyOz化合物，当采用雾化方法时，CBS-APNO / G3 / G4方法的组合显示出非常强大的功能，并且能够将ATcT再现到“近化学精度”以内，且2RMSD（近似于95％置信区间）的CxHyOz自由基计算值为0.0 +/- 4.34 kJ mol（-1），对于闭壳CxHyOz化合物计算为0.0 +/- 4.22 kJ mol（-1），总不确定度为0.0 +/-随后考虑所有85个CxHyOz化合物计算出4.27 kJ mol（-1）。鉴于使用雾化程序时这些方法可确定形成焓，我们预计涉及这些方法的等渗反应能够实现化学准确性，如叔丁基自由基所示。我们还强调指出，尽管该自由基在气相化学中具有重要意义，但在实验和理论之间仍存在分歧。动力学，热力学和化学动力学建模者均被警告，发现流行的CBS-QB3方法的性能特别差，计算得出的2RMSD为0.0 +/- 12。

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