Inorganic halide perovskites are important optoelectronic materials known to present anharmonicity and local structural disorder. While the influence of local disorder on bulk properties has received growing attention, its impact on surface properties remains unexplored. In this work, we use density functional theory (DFT) calculations to systematically investigate the effect of local disorder on the surface properties of cubic $\mathrm{Cs}B{X}_3$ perovskites ($B$ = Sn, Pb; $X$ = I, Br, Cl). We find that introducing local disorder through polymorphous configurations lowers the surface energy, making disordered surfaces more thermodynamically favorable than their high-symmetry counterparts. Local disorder also induces a downward shift in the valence band maximum (VBM) due to the elongation of $B-X$ bonds, the tilt of ${BX}_6$ octahedra, as well as a modification of surface dipoles. We employed semilocal functionals along with DFT-1/2 quasiparticle corrections to determine the absolute band positions. Our results reveal that semilocal functionals alone fail to capture the trends in the VBM across different halide compositions, whereas the inclusion of DFT-1/2 corrections successfully restores the experimentally observed trends. Furthermore, calculations based on polymorphous structures yield VBM values in much closer agreement with experimental measurements compared to those obtained from monomorphous archetypal ordered structures. This demonstrates that explicitly accounting for the polymorphous nature of the material and employing DFT-1/2 corrections lead to a more realistic description of surface electronic properties than assuming an idealized monomorphous framework and relying on semilocal DFT. These findings reveal local disorder as a key design parameter for perovskite surfaces, opening new opportunities for tuning interfacial energetics and improving device performance.

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• Anharmonic lattice dynamics
• Energy materials
• Interface & surface thermodynamics
• Photovoltaic absorbers
• Surface reconstruction
• Perovskites
• Surfaces
• First-principles calculations