Combined DFT and wave function theory approach to excited states of lanthanide luminescent materials: A case study of LaF3:Ce3+

Abstract Lanthanide luminescent materials play key roles in modern society, but their first‐principles treatment remains a great challenge due to complex manifold of electronic excited states and the difficulty in performing excited state structural relaxations that is necessary to model luminescent properties. Herein, we propose a practical approach that combines embedded cluster model (ECM) based multi‐configurational wave function theory (WFT) and occupancy constrained density‐functional theory plus the Hubbard U correction (OC‐DFT + U ) to treat lanthanide doped luminescent materials, using LaF 3 :Ce 3+ , a typical scintillator with low symmetry, as a case study. We show that the combined approach yields accurate absorption energies with an error on the order of 200 cm −1 , but the emission energies are significantly underestimated, the origin of which is further clarified by vibrationally resolved absorption and emission spectra calculation. This work demonstrates the possibility of combining ECM‐based wave function theory and periodic DFT into a comprehensive computational scheme for lanthanide luminescent materials and highlights the limitations of the current implementation of OC‐DFT + U for excited state structural optimization.