Orbital optimization (OO) has been suggested as a way to solve some shortcomings of second-order Møller–Plesset (MP2) variants and double-hybrid density functionals (DHDFs). A closer inspection of the literature, however, shows that the only two studies on OO-DHDFs were limited to three nonempirical PBE-based functionals, which are known to be of only mediocre accuracy. Herein, we provide a more in-depth analysis of OO-DHDFs with the main focus being on main-group thermochemistry, kinetics, and noncovalent interactions. We reanalyze two PBE-based OO-DHDFs and present four new OO-DHDF variants, two of which make use of the spin-component-scaling idea in their nonlocal correlation part. We also provide a more thorough analysis of three OO-MP2 variants. After assessing more than 621 reference points, we come to the conclusion that the benefits of OO are not as straightforward as previously thought. Results heavily depend on the underlying parent method. While OO-SCS/SOS-MP2 usually provide improved results—including for noncovalently bound systems—the opposite is true for OO-MP2. OO-DHDFs, like their nonoptimized counterparts, still require London-dispersion corrections. Among the DHDFs, the largest effect of OO on thermochemical properties is seen for PBE0-2 and the smallest for PBE0-DH. However, results can both worsen and improve with OO. If the latter is the case, the resulting OO-DHDF is still outperformed by the currently most accurate conventional DHDFs, namely DSD-BLYP and DSD-PBEP86. We therefore recommend the OO technique only to be used in specialized cases. For the general method user we re-emphasize using conventional dispersion-corrected DHDFs for robust, reliable results. Our findings also indicate that entirely different strategies seem to be required in order to obtain a substantial improvement over the currently best DHDFs.

中文翻译：

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对双杂交密度泛函中轨道优化在热化学，动力学和非共价相互作用处理中的有效性的综合评估

有人提出通过轨道优化（OO）来解决二阶Møller-Plesset（MP2）变体和双杂交密度泛函（DHDF）的某些缺点。然而，对文献的仔细检查表明，关于OO-DHDF的仅有两项研究仅限于三种非经验性的基于PBE的功能，而这些功能仅具有中等的准确性。在本文中，我们提供了对OO-DHDF的更深入的分析，重点是主族的热化学，动力学和非共价相互作用。我们重新分析了两个基于PBE的OO-DHDF，并提出了四个新的OO-DHDF变体，其中两个在其非局部相关部分中利用了自旋分量缩放的思想。我们还提供了三种OO-MP2变体的更全面分析。在评估了超过621个参考点之后，我们得出的结论是，OO的好处并不像以前想象的那么简单。结果在很大程度上取决于基础父方法。尽管OO-SCS / SOS-MP2通常可以提供更好的结果（包括非共价键结合的系统），但OO-MP2则相反。OO-DHDF与未优化的同类产品一样，仍需要伦敦色散校正。在DHDF中，OO对PBE0-2的热化学性质影响最大，而对PBE0-DH的影响最小。但是，使用OO会导致结果恶化和改善。如果是后者，则生成的OO-DHDF仍将比目前最准确的常规DHDF（DSD-BLYP和DSD-PBEP86）表现更好。因此，我们建议仅在特殊情况下使用OO技术。对于一般方法的用户，我们再次使用常规的色散校正DHDF进行强调，以获得可靠，可靠的结果。我们的发现还表明，似乎需要完全不同的策略才能对目前最好的DHDF进行实质性改进。