Small David W
Molecular Graphics and Computation Facility, College of Chemistry, University of California, Berkeley, California 94720, United States.
J Chem Theory Comput. 2020 Jul 14;16(7):4014-4020. doi: 10.1021/acs.jctc.0c00244. Epub 2020 Jun 26.
The focus of this work is OS-CCSD-SPT(2), which is a second-order similarity transformed perturbation theory correction to opposite spin coupled cluster singles doubles, where in the latter the same-spin amplitudes are removed and the opposite-spin ones are solved self-consistently. OS-CCSD-SPT(2) is free of empirical parameters, has an instrinsic scaling of (), and makes no use of triples. We demonstrate that, for non-multireference molecules, OS-CCSD-SPT(2) produces relative energies whose accuracy is significantly higher than what is generally expected of a triples-free model. For example, using PBE0 orbitals in the reference, OS-CCSD-SPT(2) exhibits a mean absolute deviation (MAD) of 1.13 kcal/mol with respect to CCSD(2) benchmark values for the non-multireference subset of W4-08 atomization energies (cf. a MAD > 6.5 kcal/mol for CCSD) and a MAD of 0.68 kcal/mol for the energies of reactions generated from the W4-08 molecules. These MADs are reduced to 0.61 and 0.63 kcal/mol, respectively, by a simple one-parameter spin-component scaling of the OS-CCSD-SPT(2) same-spin correlation energy. OS-CCSD is also naturally amenable to higher order corrections: the associated third-order correction, OS-CCSD-SPT(3), which does involve connected triples and quadruples, exhibits a MAD of 0.44 kcal/mol for the same atomization-energy benchmark.
这项工作的重点是OS-CCSD-SPT(2)，它是对相反自旋耦合簇单双激发态的二阶相似变换微扰理论修正，其中在后者中去除了同自旋振幅，并自洽求解相反自旋振幅。OS-CCSD-SPT(2)没有经验参数，具有内在的标度（），并且不使用三激发态。我们证明，对于非多参考分子，OS-CCSD-SPT(2)产生的相对能量的精度明显高于通常对无三激发态模型的预期。例如，在参考中使用PBE0轨道时，对于W4-08原子化能的非多参考子集，OS-CCSD-SPT(2)相对于CCSD(2)基准值的平均绝对偏差（MAD）为1.13 kcal/mol（相比之下，CCSD的MAD>6.5 kcal/mol），对于由W4-08分子产生的反应能量，MAD为0.68 kcal/mol。通过对OS-CCSD-SPT(2)同自旋相关能进行简单的单参数自旋分量标度，这些MAD分别降至0.61和0.63 kcal/mol。OS-CCSD也自然适用于高阶修正：相关的三阶修正OS-CCSD-SPT(3)，它确实涉及连通三激发态和四激发态，对于相同的原子化能基准，其MAD为0.44 kcal/mol。
