  Raman spectroscopy is an advantageous method for studying the local structure
of materials, but the interpretation of measured spectra is complicated by the
presence of oblique phonons in polycrystals of polar materials. Whilst group
theory considerations and standard ab initio calculations are helpful, they are
often valid only for single crystals. In this paper, we introduce a method for
computing Raman spectra of polycrystalline materials from first principles. We
start from the standard approach based on the (Placzek) rotation invariants of
the Raman tensors and extend it to include the effect of the coupling between
the lattice vibrations and the induced electric field, and the electro-optic
contribution, relevant for polar materials like ferroelectrics. As exemplified
by applying the method to rhombohedral BaTiO3, AlN, and LiNbO3, such an
extension brings the simulated Raman spectrum to a much better correspondence
with the experimental one. Additional advantages of the method are that it is
general, permits automation, and thus can be used in high-throughput fashion.
