Department of Organic Chemistry, Weizmann Institute of Science, IL-76100 Rechovot, Israel.
J Phys Chem A. 2009 Oct 29;113(43):11974-83. doi: 10.1021/jp903640h.
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) contributions 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 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, gef(T) identical with -[G(T) - H(0)]/T, of these alkanes. These are essential for accurate computed heats of formation of especially the larger species as the corrections for these are several times the expected uncertainty in modern computational thermochemistry methods such as W4 theory. While H(298) - H(0) is only moderately sensitive to the level of theory, gef(T) exhibits more pronounced sensitivity. Once again, double hybrids acquit themselves very well. The effects of zero-point energy and nonfactorizable rovibrational partition functions have been considered.
正丁烷、正戊烷和正己烷的构象能已在 CCSD(T) 水平和基组极限或接近基组极限处进行了计算。考虑了后 CCSD(T) 贡献，发现它们不重要。由此获得的数据用于评估各种密度泛函方法的性能。双杂交泛函，如 B2GP-PLYP 和 B2K-PLYP，特别是带有小的 Grimme 型经验色散校正，能够呈现出 CCSD(T) 质量的构象能。这些被用作更大样本烷烃的“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 B3LYP、B3PW91、BLYP、PBE 和 PBE0，在没有色散校正的情况下往往会高估构象能，而 M06 系列则严重低估 GG 相互作用能。这些泛函的 Grimme 型色散校正过度校正，导致构象顺序的定性错误。所有这些泛函在构象几何形状上也存在缺陷，特别是骨架扭转角。PW6B95 以及在较小程度上的 BMK 泛函相对没有这些缺陷。进一步研究这些方法的性能，以得出这些烷烃的焓函数，H(298)-H(0)，和吉布斯能函数，ge(T)等同于-[G(T)-H(0)]/T，的构象集合校正。这些对于准确计算特别是较大物种的生成热至关重要，因为这些校正值是现代计算热化学方法（如 W4 理论）中预期不确定性的数倍。虽然 H(298)-H(0)仅对理论水平中度敏感，但 gef(T)表现出更明显的敏感性。再次，双杂交表现非常出色。考虑了零点能和不可分解的 rovibrational 配分函数的影响。
