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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Heats of Formation of Alkali and Alkaline Earth Oxides and Hydroxides: Some Dramatic Failures of the G2 Method
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Heats of Formation of Alkali and Alkaline Earth Oxides and Hydroxides: Some Dramatic Failures of the G2 Method

机译：碱和碱土金属氧化物和氢氧化物的形成热：G2方法的一些重大失败

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High-level ab initio calculations with a variety of G2-based methods have been used to determine the structures and heats of formation of the alkali and alkaline earth oxides and hydroxides (M_2O, MOH with M = Li, Na, and K; M'O, M'(OH)_2 with M' = Be, Mg, and Ca). Standard G2 theory, which is normally very reliable for the prediction of molecular thermochemistry, is shown to be quite unsuitable for the prediction of the heats of formation of several of these highly polar species, with errors greater than 100 kJ mol~(-1) in some cases. Our calculations confirm that for systems containing the third-row atoms K and Ca, it is essential to include the 3s and 3p orbitals in the correlation space. Interestingly, an analogous relaxed-inner-valence (denoted riv) procedure is more beneficial for the Li- and Be-containing oxides and hydroxides than for the Na- and Mg-containing molecules. Inclusion of all orbitals in the correlation space (denoted full) generally provides only a slight further improvement to the results. Removal of the additivity approximation of standard G2 theory through direct large basis set QCISD(T) calculations [e.g., G2(dir,full)] has a large effect for the oxides CaO and K_2O. The QCISD(T) component of the G2 energy is poorly described for CaO, Na_2O, and K_2O, but this can be rectified through replacement of QCISD(T) with CCSD(T) [e.g., G2[CC](dri,full)]. For five molecules (CaO, Be(OH)_2, Mg(OH)_2, and K_2O) where significant discrepancies (10-30 kJ mol~(-1)) remain between the best theoretical heats of formation (i.e., G2[CC](dir,full)) and experimental values, we suggest that experimental reexamination is desirable. Structures determined at the MP2/6-311+G(3df,2p) level are in good agreement with available experimental data. Structures obtained at the standard MP2/6-31G(d) level of G2 theory are not as good, but the impact of using the simpler geometries on calculated heats of formation is generally relatively small.
机译：已使用各种基于G2的方法进行了从头算的高级从头计算，来确定碱金属和碱土金属氧化物和氢氧化物（M_2O，MOH，M = Li，Na和K; M' O，M'（OH）_2，其中M'= Be，Mg和Ca）。标准G2理论通常对分子热化学的预测非常可靠，但已证明非常不适合预测这些高极性物质中的几种的形成热，其误差大于100 kJ mol〜（-1）。在某些情况下。我们的计算结果证实，对于包含第三行原子K和Ca的系统，必须在相关空间中包含3s和3p轨道。有趣的是，与含Na和Mg的分子相比，类似的弛豫内价（表示为riv）程序对含Li和Be的氧化物和氢氧化物更有利。通常将所有轨道包含在相关空间中（表示为满），只会对结果产生一点点进一步的改善。通过直接的大基础集QCISD（T）计算[例如G2（dir，full）]消除标准G2理论的可加性近似对氧化物CaO和K_2O的影响很大。对于CaO，Na_2O和K_2O，G2能量的QCISD（T）成分描述不清，但是可以通过用CCSD（T）代替QCISD（T）来纠正[例如，G2 [CC]（dri，full） ]。对于五个分子（CaO，Be（OH）_2，Mg（OH）_2和K_2O），在最佳理论形成热（即G2 [CC]之间仍存在显着差异（10-30 kJ mol〜（-1）） ]（dir，full））和实验值，我们建议进行实验重新检查是可取的。在MP2 / 6-311 + G（3df，2p）水平确定的结构与可用的实验数据高度吻合。在G2理论的标准MP2 / 6-31G（d）级别获得的结构不是那么好，但是使用更简单的几何形状对计算出的地层热量的影响通常相对较小。

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