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

Accurate <i>ab initio</i> 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^<i>n</i>-15d¹ → 4f^<i>n</i>-based broad-band emitting lanthanoid ions such as Ce³⁺ (<i>n</i> = 1) or Eu²⁺ (<i>n</i> = 7) is the shrinkage of the metal-ligand bond length in the lowest excited 4f^<i>n</i>-15d¹ state. In this work, we use the model system SrCl₂:Eu²⁺, 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.