  The equilibrium ground-state structure of LiNbO3 in the paraelectric and
ferroelectric phases is fully optimized in a first-principles calculation using
the full-potential linearized augmented plane wave method. The equilibrium
volume, c/a ratio and all (four, in the ferroelectric phase) internal
parameters are found to be in good agreement with the experimental data. Frozen
phonon calculations are performed for TO-Gamma phonons corresponding to the A1
and A2 irreducible representations of the R3c space group in the ferroelectric
phase. The comparison with available experimental frequencies for the A1 modes
is satisfactory (including the 6Li isotope effect), and the displacement
patterns are unambiguously attributed. For the (Raman inactive) A2 modes,
phonon frequencies and eigenvectors are predicted.
