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This paper extends the diagrammatic Multiplet-Sum Method (MSM) within Density-Functional Theory (DFT) to incorporate nondynamic correlation effects, addressing a limitation of common DFT approximations. The key innovation is the inclusion of relaxation effects via nonorthogonal configuration interaction (NOCI) within the two-orbital two-electron model (TOTEM). Application to the LiH molecule demonstrates accurate ground-state potential energy curve prediction, particularly at the ionic-to-open-shell-singlet avoided crossing.
Overcoming a key limitation of standard DFT, this work shows how to accurately model challenging chemical systems like LiH by incorporating relaxation effects into the Multiplet-Sum Method.
Ideal density-functional approximations (DFAs) should account for dynamic, static, and nondynamic correlation. While common DFAs struggle with the latter two, the Ziegler-Rauk-Baerends-Daul multiplet sum method (MSM) provides a pragmatic way to include static correlation. In this article, we use diagrammatic MSM density-functional theory (diag MSM DFT) using the two-orbital two-electron model (TOTEM) to extend MSM DFT to include nondynamic correlation without relying on symmetry arguments. Building on previous formulations [A. Ponra, C. Bakasa, A.J. Etindele,and M.E. Casida, J. Chem. Phys. 159, 244306 (2023); M.E. Casida, A. Ponra, C. Bakasa, and A.J.Etindele, J. Chem. Phys. 162, 144317 (2025)] that lacked relaxation effects, this article incorporates relaxation via nonorthogonal configuration interaction (NOCI). We demonstrate that this modified diag MSM DFT produces an accurate ground-state potential energy curve (PEC) for lithium hydride (LiH), even at the ionic-to-open-shell-singlet avoided crossing characterized by significant charge transfer. This encouraging result suggests that the model can be extended to (at least) other singly and multiply-bonded diatomic molecules, while providing insight into a novel way to include strong correlation in DFT.
