Density functional study of the electronic and vibrational properties of TiOCl

We present the phonon spectrum of TiOCl computed using hybrid density functional theory (DFT). A complete analysis of the spectrum is performed for the space group Pmmn (high symmetry phase) and the space group P2_1/m (low symmetry phase), which is the symmetry of the spin-Peierls phase. We show that the nonlocal correlations present in the hybrid DFT approach are important for understanding the electron-lattice interactions in TiOCl. The computed frequencies compare well with those observed in Raman and infrared spectroscopy experiments and we identify the origin of an anomalous phonon observed in Raman spectroscopy. The relationship between relevant zone boundary phonons in the high symmetry phase and the zone center counterparts in the P2_1/m symmetry allow us to speculate about the origin of the spin-Peierls phonon.