Department of Physical Chemistry and Materials Science, Budapest University of Technology and Economics, Budapest P.O. Box 91, H-1521 Hungary.
J Phys Chem A. 2011 Jul 7;115(26):7765-72. doi: 10.1021/jp2020879. Epub 2011 Jun 7.
The currently accepted D(0)((35)Cl(2)) is 239.221 ± 0.001 kJ/mol, whereas popular theoretical model chemistries provide values in the range of 233-247 kJ/mol, and even the so-called high-accuracy protocols can yield values as low as 237.9 kJ/mol and as high as 240.1 kJ/mol for D(0)((35)Cl(2)). The aim of this study was to uncover the sources of error inherent in the theoretical approaches. Therefore, a coupled-cluster-based composite model chemistry was utilized that included contributions of up to pentuple excitations, as well as corrections beyond the nonrelativistic and Born-Oppenheimer approximations. In our calculations, correlation consistent basis set families were used up to octuple-ζ basis sets. It was found that the following factors, in order of significance, can be identified as the most important error sources: (i) the considerably large relativistic contributions carrying large uncertainties, (ii) the very slow convergence of the Møller-Plesset (MP2) correlation energy (with the octuple-ζ basis set, it still contains an error of a few tenth of a kJ/mol), (iii) the slow convergence of the coupled-cluster singles and doubles (CCSD) contribution (it needs a octuple-ζ basis set to converge within 0.1 kJ/mol), and (iv) the relatively large basis set (quadruple-ζ) needed in the calculation of an accurate perturbative quadruples contribution. It is also notable that, for chlorine, the use of a quintuple-ζ basis set for the Hartree-Fock energy, the MP2 correlation energy, and for the CCSD and perturbative triples contributions, which is the usual treatment in almost every high-accuracy model chemistry, resulted in the overestimation of all of these contributions (altogether about by 1.8 kJ/mol). However, this overestimation is accidentally compensated by (i) using an inappropriate, small basis set for the valence electron contribution due to quadruple excitations (∼1.2 kJ/mol), (ii) neglecting the effects of core electron contributions due to quadruple excitations (∼0.2 kJ/mol), and (iii) neglecting relativistic effects beyond the scalar relativistic treatment (∼0.3 kJ/mol). The most reliable theoretical estimate for D(0)((35)Cl(2)) obtained in this study, 239.27 ± 1.30 kJ/mol, differs by only 0.05 kJ/mol from the most accurate experimental result. This study also underpins the effect of relativistic contributions, which precludes current model chemistries to enter the range of sub-kJ/mol accuracy for second-row systems.
目前公认的 D(0)((35)Cl(2)) 值为 239.221 ± 0.001 kJ/mol，而流行的理论化学模型提供的值在 233-247 kJ/mol 范围内，即使是所谓的高精度协议也能产生低至 237.9 kJ/mol 和高至 240.1 kJ/mol 的 D(0)((35)Cl(2)) 值。本研究的目的是揭示理论方法固有的误差源。因此，使用了一种基于耦合簇的组合模型化学，其中包括高达五重激发的贡献，以及超出非相对论和 Born-Oppenheimer 近似的修正。在我们的计算中，使用了关联一致的基组族，最高可达八重基组。结果发现，以下因素按重要性顺序可被确定为最重要的误差源：(i) 相对论贡献相当大，带有很大的不确定性，(ii) Møller-Plesset (MP2) 相关能量的收敛非常缓慢（使用八重基组，它仍然包含几十分之一 kJ/mol 的误差），(iii) 耦合簇单双贡献的收敛缓慢（需要八重基组才能在 0.1 kJ/mol 内收敛），以及 (iv) 准确计算扰动四元贡献所需的相对较大的基组（四重基组）。值得注意的是，对于氯，在几乎每一种高精度模型化学中，使用 Hartree-Fock 能量的五重基组、MP2 相关能量以及 CCSD 和扰动三的贡献，都导致了所有这些贡献的高估（总共约 1.8 kJ/mol）。然而，这种高估被以下因素意外地补偿了：(i) 由于四重激发，对价电子贡献使用不适当的小基组（约 1.2 kJ/mol），(ii) 忽略了由于四重激发引起的芯电子贡献的影响（约 0.2 kJ/mol），以及 (iii) 忽略了标量相对论处理之外的相对论效应（约 0.3 kJ/mol）。本研究获得的 D(0)((35)Cl(2)) 的最可靠理论估计值为 239.27 ± 1.30 kJ/mol，仅比最准确的实验结果相差 0.05 kJ/mol。这项研究还支持了相对论贡献的影响，这使得当前的模型化学无法进入第二行系统亚千焦/摩尔精度的范围。
