CnH2n +2烷烃异构体（n=2-8）构象平衡的从头算和密度泛函方法研究
Performance of ab initio and density functional methods for conformational equilibria of CnH2n+2 alkane isomers (n=2-8)
摘要：
在CCSD（T）水平上计算了正丁烷、正戊烷和正己烷的构象能，并在基组极限或接近基组极限。CCSD（T）后的贡献被认为是不重要的。由此获得的数据被用来评估各种密度泛函方法的性能。像B2 GP-PLYP和B2 K-PLYP这样的双杂化泛函，特别是在小的Grimme型经验色散校正下，能够呈现CCSD（T）质量的构象能。然后，这些被用作较大烷烃样本的“二级标准”，包括异戊烷和支链己烷以及庚烷和辛烷的关键异构体。流行的DFT泛函，如B3 LYP，B3 PW 91，BLYP，PBE和PBE 0往往高估构象能量没有色散校正，而M06家族严重低估GG相互作用能。Grimme型色散校正这些overcorrect，并导致定性错误的构象排序。所有这些功能也表现出不足之处的构象的几何形状，特别是骨干扭转角。PW 6 B 95和BMK泛函在较小程度上相对没有这些缺陷。这些方法的性能进行了进一步研究，以获得构象合奏修正的焓函数，$H_{298}-H_0$，和吉布斯能函数，${\rm gef}(T)\equiv - [G(T)-H_0]/T$，这些烷烃。虽然$H_{298}-H_0$对理论水平仅具有中等敏感性，但${\rm gef}(T)$表现出更显著的敏感性。再一次，双杂交体表现得非常好。
英文摘要：
Conformational energies of n-butane, n-pentane, and n-hexane have been calculated at the CCSD(T) level and at or near the basis set limit. Post-CCSD(T) contribution were considered and found to be unimportant. The data thus obtained were used to assess the performance of a variety of density functional methods. Double-hybrid functionals like B2GP-PLYP and B2K-PLYP, especially with a small Grimme-type empirical dispersion correction, are capable of rendering conformational energies of CCSD(T) quality. These were then used as a `secondary standard' for a larger sample of alkanes, including isopentane and the branched hexanes as well as key isomers of heptane and octane. Popular DFT functionals like B3LYP, B3PW91, BLYP, PBE, and PBE0 tend to overestimate conformer energies without dispersion correction, while the M06 family severely underestimates GG interaction energies. Grimme-type dispersion corrections for these overcorrect and lead to qualitatively wrong conformer orderings. All of these functionals also exhibit deficiencies in the conformer geometries, particularly the backbone torsion angles. The PW6B95 and, to a lesser extent, BMK functionals are relatively free of these deficiencies. Performance of these methods is further investigated to derive conformer ensemble corrections to the enthalpy function, $H_{298}-H_0$, and the Gibbs energy function, ${\rm gef}(T)\equiv - [G(T)-H_0]/T$, of these alkanes. While $H_{298}-H_0$ is only moderately sensitive to the level of theory, ${\rm gef}(T)$ exhibits more pronounced sensitivity. Once again, double hybrids acquit themselves very well.
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