The Raman spectrum of Pyrope garnet. A Quantum Mechanical Simulation of Frequencies, Intensities and Isotope Shifts

The Raman spectrum of pyrope garnet is simulated in ab initio quantum-mechanical calculations, using an all electron Gaussian-type basis set and the hybrid B3LYP functional. Frequencies calculated for the 25 Raman active modes are in excellent agreement with the several sets of experimental data with the mean absolute difference ranging from 4 to 8 cm−1. Comparison of the computed and experimental spectrum shows excellent agreement for most of the intensities as well. Modes missing from experiment are shown to be characterized by low (computed) intensity. Spurious peaks in the experimental spectra are also identified. The isotopic effect has been simulated for 24Mg → 26Mg substitution and shows and excellent agreement with shifts reported in one of the experiments. Agreement is excellent for all but one mode, which turns out to be attributed to the wrong symmetry in the experiment.