Ab Initio Modeling of the Photoluminescence of SrCl2:Eu2+─Direct Comparison between Density Functional and Wave Function Theory-Based Approaches

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Accurate ab initio quantum chemical approaches for the description of two-open-shell centers in periodic systems, such as lanthanoid ions in inorganic host matrices, remain a challenging topic in computational chemistry. In contrast, experimental high-quality spectroscopic data on selected lanthanoid-activated inorganic phosphors are widely available and offer a perfect ground to test new theoretical developments. One particular benchmark of computational methods in the field of 4f n -1 5d 1 → 4f n -based broad-band emitting lanthanoid ions such as Ce 3+ ( n = 1) or Eu 2+ ( n = 7) is the shrinkage of the metal–ligand bond length in the lowest excited 4f n -1 5d 1 state. In this work, we use the model system SrCl 2:Eu 2+, which crystallizes in a fluorite-type structure with cubically coordinated Sr sites, as a case study for bond length contraction upon excitation─a behavior contrary to the usual bond-length increase (or breaking) encountered in most excited states with antibonding character. We performed a series of calculations on model systems ranging from realistic periodic models to idealized molecular clusters. We compare wavefunction theory- to density functional theory-based approaches, offering guidelines on how to evaluate these methodologies for their suitability as predictive tools for the photoluminescence properties of lanthanoid-activated phosphors.