Halide perovskites have emerged as highly promising materials for photovoltaic applications. However, their ionic character and structurally soft lattices give rise to complex dynamical phenomena, including ion migration, halide segregation, structural fluctuations, and degradation under operational stresses such as illumination, elevated temperature, and electrical bias. Molecular passivation of perovskite surfaces, interfaces, and bulk regions is widely employed to mitigate these instabilities. Nevertheless, the atomistic mechanisms through which passivation species suppress degradation, regulate ion migration, and modify the interfacial electronic structure remain incompletely understood. In this project, we will employ first-principles electronic-structure calculations and ab initio molecular dynamics simulations to investigate the structural, electronic, dynamical, and spectroscopic properties of halide perovskites and their interactions with passivation molecules. The simulations will examine how temperature, photoinduced effects, defects, and molecular passivation influence lattice dynamics, ionic motion, interfacial organization, structural stability, and electronic properties. Calculated structural, electronic, and spectroscopic observables will be compared with experimental measurements to facilitate their atomistic interpretation. The resulting theoretical framework will complement ongoing experimental studies, clarify the mechanisms governing perovskite degradation and passivation, and guide the design of future materials and experiments. This six-month small allocation will serve as an initial pilot phase of a broader computational research objective. It will be used to benchmark and establish the required computational workflows; construct and validate representative passivation-molecules and perovskite/passivation-molecule models; and evaluate the computational cost and scaling of the planned simulations to generate structural, dynamical, and spectroscopic results. These outcomes will define the experimentally relevant systems and the ideal simulation setup to help plan a subsequent NAISS medium allocation. The follow-up application will extend the calculations to supercell slab models mimicking the experiments, longer molecular-dynamics trajectories, including different temperature ranges as well as electric field, and a systematic comparison of multiple passivation strategies in close coordination with experimental investigations.