<p>This dataset contains density functional theory (DFT) calculation files supporting the structural, electronic, and property analysis of the In₂SeS₄ spinel-type phase, derived from annealing layered InSe crystals in sulfur vapour. The computational work accompanies a study aimed at validating the reported Fd-3m structural model, where selenium occupies the tetrahedral site normally taken by a divalent cation. </p> <p>The repository includes the fully relaxed crystal structure, Quantum ESPRESSO (v7.5) input and output files for structural optimization, self-consistent field (SCF) calculations, non-self-consistent field (NSCF) electronic structure calculations, and project wavefunctions for the density of states (DOS) and partial density of states (PDOS). Calculations were performed using the PBE exchange-correlation functional. The structural relaxation reproduces the internal sulfur coordinate within 0.0002 and local coordination within 0.1 degrees compared to X-ray refinement data, confirming the internal consistency of the geometric model. Electronic structure calculations reveal a finite density of states at the Fermi level and the absence of a band gap within the PBE framework, showing that the unusual site occupation is reflected clearly in the ordering of the electronic states (with substantial Se 4p and In 5s contributions above the upper sulfur-derived occupied states).</p> <p>The workflow and data organization follow the guidelines for data reproducibility, providing the raw calculation parameters and numerical data extracted for the publication figures in the corresponding paper.</p>