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.