  The Born effective charge Z^{*} and dielectric tensor \epsilon_{\infty} of
KNbO_3 are found to be very sensitive to the atomic geometry, changing by as
much as 27% between the paraelectric cubic and ferroelectric tetragonal and
rhombohedral phases. Subtracting the bare ionic contribution reveals changes of
the dynamic component of Z^{*} as large as 50%, for atomic displacements that
are typically only a few percent of the lattice constant. Z^{*},
\epsilon_{\infty} and all phonon frequencies at the Brillouin zone center were
calculated using the {\it ab initio} linearized augmented plane-wave linear
response method with respect to the reference cubic, experimental tetragonal,
and theoretically determined rhombohedral ground state structures. The ground
state rhombohedral structure of KNbO_3 was determined by minimizing the forces
on the relaxed atoms. By contrast with the cubic structure, all zone center
phonon modes of the rhombohedral structure are stable and their frequencies are
in good agreement with experiment. In the tetragonal phase, one of the soft
zone center modes in the cubic phase is stablized. In view of the small atomic
displacements involved in the ferroelectric transitions, it is evident that not
only the soft mode frequencies but also the Born effective charge and
dielectric constants are very sensitive to the atomic geometry.
