Soydaş Emine, Bozkaya Uğur
Department of Chemistry, Atatürk University, Erzurum, 25240, Turkey.
J Comput Chem. 2014 May 30;35(14):1073-81. doi: 10.1002/jcc.23592. Epub 2014 Mar 26.
An assessment of the orbital-optimized coupled-electron pair theory [or simply "optimized CEPA(0)," OCEPA(0)] [Bozkaya and Sherrill, J. Chem. Phys. 2013, 139, 054104] for thermochemistry and kinetics is presented. The OCEPA(0) method is applied to closed- and open-shell reaction energies, barrier heights, and radical stabilization energies (RSEs). The performance of OCEPA(0) is compared with those of the MP2, CEPA(0), OCEPA(0), CEPA(1), coupled-cluster singles and doubles (CCSD), and CCSD(T) methods [at complete basis set limits employing cc-pVTZ and cc-pVQZ basis sets]. For the most of the test sets, the OCEPA(0) method performs better than CEPA(0), CEPA(1), and CCSD, and provides accurate results. Especially, for open-shell reaction energies and barrier heights, the OCEPA(0)-CEPA(1) and OCEPA(0)-CCSD differences become obvious. Similarly, for barrier heights and RSEs, the OCEPA(0) method improves on CEPA(0) by 1.6 and 2.3 kcal mol(-1) . Our results demonstrate that the CEPA(0) method dramatically fails when the reference wave function suffers from the spin-contamination problem. Conversely, the OCEPA(0) method can annihilate spin-contamination in the unrestricted-Hartree-Fock initial guess orbitals and can yield stable solutions. For overall evaluation, we conclude that the OCEPA(0) method is quite helpful not only for problematic open-shell systems and transition-states but also for closed-shell molecules. Hence, one may prefer OCEPA(0) over CEPA(0), CEPA(1), and CCSD as an O(N6) method, where N is the number of basis functions, for thermochemistry and kinetics. As discussed previously, the cost of the OCEPA(0) method is as much as of CCSD and CEPA(1) for energy computations. However, for analytic gradient computations, the OCEPA(0) method is two times less expensive than CCSD and CEPA(1). Further, the stationary properties of the OCEPA(0) method making it promising for excited state properties via linear response theory.
本文对用于热化学和动力学的轨道优化耦合电子对理论[或简称为“优化CEPA(0)”，即OCEPA(0)] [博兹卡亚和谢里尔，《化学物理杂志》，2013年，第139卷，054104]进行了评估。OCEPA(0)方法被应用于闭壳层和开壳层反应能量、势垒高度以及自由基稳定能（RSEs）。将OCEPA(0)的性能与MP2、CEPA(0)、OCEPA(0)、CEPA(1)、耦合簇单双激发（CCSD）以及CCSD(T)方法的性能进行了比较[在使用cc-pVTZ和cc-pVQZ基组的完备基组极限下]。对于大多数测试集，OCEPA(0)方法比CEPA(0)、CEPA(1)和CCSD表现更好，并能提供准确的结果。特别是，对于开壳层反应能量和势垒高度，OCEPA(0)与CEPA(1)以及OCEPA(0)与CCSD的差异变得明显。同样，对于势垒高度和RSEs，OCEPA(0)方法比CEPA(0)分别提高了1.6和2.3千卡/摩尔。我们的结果表明，当参考波函数存在自旋污染问题时，CEPA(0)方法会显著失效。相反，OCEPA(0)方法可以消除无限制Hartree-Fock初始猜测轨道中的自旋污染，并能产生稳定的解。总体评估得出，我们认为OCEPA(0)方法不仅对有问题的开壳层系统和过渡态，而且对闭壳层分子都非常有帮助。因此，作为一种O(N6)方法（其中N是基函数的数量），在热化学和动力学方面，相对于CEPA(0)、CEPA(1)和CCSD，人们可能更倾向于使用OCEPA(0)。如前所述，对于能量计算，OCEPA(0)方法的成本与CCSD和CEPA(1)相当。然而，对于解析梯度计算，OCEPA(0)方法的成本比CCSD和CEPA(1)低两倍。此外，OCEPA(0)方法的平稳性质使其有望通过线性响应理论用于激发态性质的研究。
