Local density and generalized gradient approximation studies of KNbO3 and BaTiO3

Density functional total energy and electronic structure calculations for KNbO3 and BaTiO3 using a generalized gradient approximation (GGA) functional are reported. The results are contrasted with those of parallel local density approximation (LDA) calculations. For KNbO3 a larger equilibrium volume in considerably better agreement with experiment is obtained with the GGA functional. Neither the electronic structure, the energetics of the ferroelectric mode at fixed volume, nor the volume dependence of the ferroelectric instability were substantially changed from their LDA values. In BaTiO3 the GGA functional also yields an increased equilibrium volume relative to the LDA. However, in contrast to KNbO3 the volume is over-corrected, leading to little if any improvement. Calculations for BaO were performed in order to understand the origins of this overcorrection. A similar overestimation of the equilibrium volume in BaO was obtained.