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While reading the literature on the absorption spectra of materials from the MPX3 group (M= Mn,Ni,Co, Fe; layered antiferromagnets) I come across expressions such as d-d transitions, which are visible as weak transitions before the absorption edge (example DOI). These formulations and designations, as I understand it, come from the molecular theories of the crystal field. I calculated (DFT+U) the density of states for such materials and I do not see separated bands in the electronic structure composed of only d-states (which is not surprising), only bands that, for example, consist of half of the p-ligand states and the d-states of the transition metal. Nowadays, in the literature on these crystalline materials, such peaks in the spectra are called d-d transitions that lie in the energy gap.

What are these d-d transitions in a pure crystal? Can they be captured with DFT? In addition to the electronic structure of the crystal, do I need to consider any internal atomic levels of the transition metal itself in this case?

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    $\begingroup$ In molecules, d-d transitions can be computed by TDDFT, but they are generally inaccurate, and multireference methods are generally necessary. I guess in crystals the situation is even worse (for example the self-interaction error of density functionals is more pronounced), but I don't have experience on this $\endgroup$
    – wzkchem5
    Aug 3, 2023 at 14:01

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