The infrared spectrum of spessartine Mn3Al2Si3O12: An ab initio all electron simulation with five different functionals (LDA, PBE, PBESOL, B3LYP and PBE0)

► Calculation of IR intensities of spessartine, a complex system (large primitive cell and high symmetry). ► Calculation of the full reflectance spectrum allows a detailed comparison with experimental data. ► Study of the effect of the Hamiltonian, an issue of intense research and debate in this period. ► Hybrid density functionals were also considered, which usually are avoided due to the higher complexity of the Hartree-Fock exchange contribution. These functional, in particular B3LYP, are shown to perform much better than other approximations such as LDA and GGA.