  In order to clarify the physics of the crossover from a Peierls band
insulator to a correlated Mott-Hubbard insulator, we analyze ground-state and
spectral properties of the one-dimensional half-filled Holstein-Hubbard model
using quasi-exact numerical techniques. In the adiabatic limit the transition
is connected to the band to Mott insulator transition of the ionic Hubbard
model. Depending on the strengths of the electron-phonon coupling and the
Hubbard interaction the transition is either first order or evolves
continuously across an intermediate phase with finite spin, charge, and optical
excitation gaps.
