Applied Physical Chemistry, KTH Royal Institute of Technology, Stockholm, Sweden.
Phys Chem Chem Phys. 2013 Apr 14;15(14):5108-14. doi: 10.1039/c3cp44319a.
This work discusses the dependence of transition state geometries on the choice of quantum chemical optimization method for the extensively studied Diels-Alder reaction. Rather significant differences are observed between post-Hartree-Fock methods and (hybrid) density functional theory, where the latter predicts larger asynchronicities. The results show that the low MP2 asynchronicity observed is likely artificial. Still, there are significant discrepancies between hybrid and pure density functionals. The role of the exchange functional seems to be most prominent in less activated reacting systems, while the importance of the correlation functional seems to increase as they become more activated by, e.g., an electron-donating group on the diene. To correct the dubious MP2 geometries, we employed the SCS-MP2 protocol for transition state optimization, which leads to significantly better results with respect to CCSD/6-31+G(d) level calculations. We conclude that in order for hybrid functionals to give descriptions consistent with the sample post-Hartree-Fock methods, a balanced combination of both Hartree-Fock exchange (with a couple of exceptions) and a well-behaved correlation functional is required. Given that the benchmark CCSD/6-31+G(d) geometries are sufficient representations, the best geometries were obtained using ωB97X(D), B2PLYP(D) and M06-2X.
这项工作讨论了过渡态几何形状对广泛研究的 Diels-Alder 反应的量子化学优化方法选择的依赖性。后 Hartree-Fock 方法和（混合）密度泛函理论之间存在相当大的差异，后者预测更大的异步性。结果表明，观察到的低 MP2 异步性可能是人为的。尽管如此，混合和纯密度泛函之间仍然存在显著差异。交换函数的作用在反应性较低的体系中似乎最为突出，而随着它们被电子供体基团等供体基团激活程度的增加，相关函数的重要性似乎会增加。为了修正可疑的 MP2 几何形状，我们采用了 SCS-MP2 协议进行过渡态优化，这使得与 CCSD/6-31+G(d)水平计算相比，结果有了显著改善。我们得出的结论是，为了使混合函数能够给出与样本后 Hartree-Fock 方法一致的描述，需要平衡 Hartree-Fock 交换（除了几个例外）和良好的相关函数的组合。鉴于基准 CCSD/6-31+G(d) 几何形状是足够的表示，最好的几何形状是使用 ωB97X(D)、B2PLYP(D) 和 M06-2X 获得的。
