Department of Chemistry and Biochemistry, Auburn University, Auburn, Alabama 36849, USA.
J Chem Phys. 2019 Feb 21;150(7):074109. doi: 10.1063/1.5086079.
The recently proposed spin-flip symmetry-adapted perturbation theory (SF-SAPT) first-order exchange energy [Patkowski et al., J. Chem. Phys. 148, 164110 (2018)] enables the standard open-shell SAPT approach to treat arbitrary spin states of the weakly interacting complex. Here, we further extend first-order SF-SAPT beyond the single-exchange approximation to a complete treatment of the exchanges of electrons between monomers. This new form of the exchange correction replaces the single-exchange approximation with a more moderate single-spin-flip approximation. The newly developed expressions are applied to a number of small test systems to elucidate the quality of both approximations. They are also applied to the singlet-triplet splittings in pancake bonded dimers. The accuracy of the single-exchange approximation deteriorates at short intermolecular separations, especially for systems with few electrons and for the high-spin state of the complex. In contrast, the single-spin-flip approximation is exact for interactions involving a doublet molecule and remains highly accurate for any number of unpaired electrons. Because the single-exchange approximation affects the high-spin and low-spin states of pancake bonded complexes evenly, the resulting splitting values are of similar accuracy to those produced by the formally more accurate single-spin-flip approximation.
最近提出的自旋翻转对称自适应微扰理论 (SF-SAPT) 一阶交换能量[Patkowski 等人，J. Chem. Phys. 148, 164110 (2018)]使标准的开壳层 SAPT 方法能够处理弱相互作用复合物的任意自旋态。在这里，我们进一步将一阶 SF-SAPT 从单交换近似扩展到单体之间电子交换的完全处理。这种新形式的交换校正用更温和的单自旋翻转近似代替了单交换近似。新开发的表达式被应用于许多小测试系统，以阐明这两种近似的质量。它们也被应用于煎饼键联二聚体的单重态-三重态分裂。单交换近似在短分子间分离时精度降低，特别是对于电子数较少的系统和复合物的高自旋态。相比之下，单自旋翻转近似对于涉及二聚体的相互作用是精确的，并且对于任何数量的不成对电子仍然保持高度精确。由于单交换近似均匀地影响煎饼键联复合物的高自旋和低自旋态，因此产生的分裂值与形式上更精确的单自旋翻转近似产生的分裂值具有相似的精度。
