We study the role of non-adiabatic Holstein electron-phonon coupling on the
neutral-ionic phase transition of charge transfer crystals which can be tuned
from continuous to discontinuous, using exact numerical diagonalization. The
variation of electronic properties through the transition is smoothed by
nonadiabaticity. Lattice properties are strongly affected, and we observe both
squeezing and antisqueezing, depending on details of the adiabatic potentials,
and identify the quantum uncertainty of the phonons as the most sensitive
measure of nonadiabaticity. The adiabatic limit is regular for a continuous
transition but turns out completely inadequate near a discontinuous transition.
The relevance of coherent state approaches is assessed critically.Comment: latex manuscript (7 pages), 3 eps figures; revised version, better
discussion, one figure replaced; to be published in Europhys. Let
