Diagrammatic multiplet-sum method (MSM) density-functional theory (DFT). III. Inclusion of relaxation and application...

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 to extend MSM DFT to include nondynamic correlation without relying on symmetry arguments. Building on previous formulations [Ponra et al., J. Chem. Phys. 159, 244306 (2023) and Casida et al., J. Chem. Phys. 162, 144317 (2025)] that lacked relaxation effects, this article incorporates relaxation via nonorthogonal configuration interaction. We demonstrate that this modified diag MSM DFT produces an accurate ground-state potential energy curve for lithium hydride, 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.