  We present exact diagonalization results on a modified Peierls-Hubbard model
for the neutral-ionic phase transition. The ground state potential energy
surface and the infrared intensity of the Peierls mode point to a strong,
non-linear electron-phonon coupling, with effects that are dominated by the
proximity to the electronic instability rather than by electronic correlations.
  The huge infrared intensity of the Peierls mode at the ferroelectric
transition is related to the temperature dependence of the dielectric constant
of mixed-stack organic crystals.
