图解多重态求和法（MSM）密度泛函理论（DFT）：II. 双轨道双电子模型（TOTEM）的完备化及其在氢化锂（LiH）避免交叉中的应用
Diagrammatic Multiplet-Sum Method (MSM) Density-Functional Theory (DFT): II. Completion of the Two-Orbital Two-Electron Model (TOTEM) with an Application to the Avoided Crossing in Lithium Hydride (LiH)
AI总结：
本文将图解 MSM-DFT 扩展至完整双轨道双电子模型，通过恰当描述离子态与共价态的避免交叉，成功使 LiH 解离为中性原子，并在多种泛函下验证了方法的无参数性和耦合元的准确性。
摘要：
Ziegler-Rauk-Baerends 多重态求和法（MSM）假设密度泛函理论（DFT）能够很好地描述由单一行列式主导的态，然后利用对称性向 DFT 中加入静态关联。在我们之前的文章（第一篇）[J. Chem. Phys. 159, 244306 (2023)] 中，我们引入了图解 MSM-DFT 作为工具，以帮助扩展 MSM-DFT，使其包含即使在缺乏对称性的情况下成键和断键所需的非动态关联。该方法的一个吸引人的特点是无需引入任何依赖于泛函的参数，尽管在波函数理论（WFT）与 MSM-DFT 图解之间建立对应关系时需要做出选择。第一篇中的初步示例对双轨道双电子模型（TOTEM）的使用不如本可以做到的那样完整。本文将图解 MSM-DFT 扩展以处理完整的 TOTEM，并表明当使用图解 MSM-DFT 技术对离子键合态与共价键合态之间的避免交叉进行恰当描述时，非对称的氢化锂（LiH）分子会解离为中性原子。该方法在 Hartree-Fock 和三种泛函（LDA、PW91 和 B3LYP）上进行了测试。所有泛函都给出了相似的结果，这正是一个表述恰当的无参数理论所应预期的。与现有估计值的一致性表明，本文引入的耦合元的大小是极好的。然而，要使我们的图解 MSM-DFT 基态势能曲线与高质量从头算得到的势能曲线达到定量一致，还需要更多工作。
英文摘要：
The Ziegler-Rauk-Baerends multiplet sum method (MSM) assumes that density-functional theory (DFT) provides a good description of states dominated by a single determinant. It then uses symmetry to add static correlation to DFT. In our previous article (Article I) [J. Chem. Phys. 159, 244306 (2023)], we introduced diagrammatic MSM-DFT as a tool to aid in extending MSM-DFT to include the nondynamic correlation needed for making and breaking bonds even in the absence of symmetry. An attractive feature of this approach is that no functional-dependent parameters need to be introduced, though choices are needed in making correspondances between wave function theory (WFT) and MSM-DFT diagrams. The preliminary examples in Article I used the two-orbital two-electron model (TOTEM) less completely than could have been the case. Diagrammatic MSM-DFT is extended here to treat the full TOTEM and it is shown that the unsymmetric lithium hydride (LiH) molecule dissociates into neutral atoms when diagrammatic MSM-DFT techniques are used to introduce a proper description of the avoided crossing between ionic bonding and covalent bonding states.The method is tested for Hartree-Fock and for three functionals (LDA, PW91, and B3LYP). All the functionals yield similar results as should be expected for a properly-formulated parameter-free theory. Agreement with available estimates show that the magnitude of the coupling element introduced here is excellent. However more work will be needed to obtain quantitative agreement between our diagrammatic MSM-DFT ground-state potential energy curve and that found from high-quality ab initio calculations
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