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Hybrid organic–inorganic perovskites (HOIPs) such as methylammonium lead iodide (MAPbI _3 ) are promising candidates for use in photovoltaic cells and other semiconductor applications, but their limited chemical stability poses obstacles to their widespread use. Ab initio modeling of finite-temperature and pressure thermodynamic equilibria of HOIPs with their decomposition products can reveal stability limits and help develop mitigation strategies. We here use a previously published experimental temperature-pressure equilibrium to benchmark and demonstrate the applicability of the harmonic and quasiharmonic approximations, combined with a simple entropy correction for the configurational freedom of methylammonium cations in solid MAPbI _3 and for several density functional approximations, to the thermodynamics of MAPbI _3 decomposition. We find that these approximations, together with the dispersion-corrected hybrid density functional HSE06, yield remarkably good agreement with the experimentally assessed equilibrium between T = 326 K and T = 407 K, providing a solid foundation for future broad thermodynamic assessments of HOIP stability.