The calculations were performed using the full potential Linear Augmented Plane Waves method (see, e.g., (7)) with the the addition of local orbital basis functions (8) as implemented in WIEN97 FLAPW code (9). The exchange-correlation was treated within the local density approximation, using the parametrization by Perdew and Wang (10). The core states were treated fully relativistically, and the semicore and valence states were computed in a scalar relativistic approximation. The structure optimization in para- and ferroelectric phase, as well as frozen phonon calculations, were performed using a special k-points mesh which generated 20 k points in the irreducible Brillouin zone. We tested the convergence in the k space integration using a k-mesh (28 irreducible k-points) and found the difference in the total energy trends, as compared with the results on a sparcer 20-points k-mesh, negligible for the analysis of lattice dynamics and structure optimization.
The muffin tin radii chosen were 1.9 a.u. for Nb and 1.6 a.u. for Li and O, close to the values used by Inbar and Cohen(5) in their FLAPW calculation. The convergency of the results with respect to the number of augmented plane waves was also tested; we used, on the average, 980 basis functions for each k point. The forces acting on atoms have been calculated and used in the course of the structure optimization.