Lattice dynamics of TiO2 rutile: influence of gradient corrections in density functional calculations

Density functional calculations are performed for bulk TiO2 rutile. The equilibrium geometry, bulk modulus and the Γ-point phonons are calculated. The local density approximation (LDA) and two generalized-gradient approximations (PBE and PW91) are used to describe the exchange–correlation energy. The LDA vibrational frequencies are in excellent agreement with experiment. PBE predicts the frequency of the transverse optic (TO) A2u mode to be imaginary, leading to instability and to a ferroelectric phase transition. This result disagrees with all previous findings. The discrepancy between the PBE and LDA result is due to the larger equilibrium volume predicted by the PBE functional.