Quantum-mechanical simulation of the IR reflectance spectrum of Mn{sub 3}Al{sub 2}Si{sub 3}O{sub 12} spessartine

The reflectance spectrum of one member of the garnet family, Mn{sub 3}Al{sub 2}Si{sub 3}O{sub 12} spessartine, was computed at the ab initio level with an all electron Gaussian type basis set and the B3LYP Hamiltonian. The static high frequency dielectric constant was obtained by applying the Coupled Perturbed Kohn Sham scheme as implemented in the CRYSTAL code; the Hessian matrix was evaluated numerically starting from the analytical gradients of the total energy with respect to the Cartesian coordinates of the atoms; the oscillator strengths were computed from well localized Wannier functions. An excellent agreement was obtained with the corresponding experimental spectrum, the exception being the very low frequency region.