  We investigate the ground state phase diagram of the one-dimensional ionic
Hubbard model with an alternating potential at half-filling by the bosonization
technique as well as by numerical diagonalization of finite systems with the
Lanczos and DMRG methods. Our results support the existence of a single
"metallic" transition point from a band to a correlated insulator with
simultaneous charge and bond-charge order. In addition, we present results for
the optical conductivity obtained by the dynamical DMRG method. The
insulator-insulator phase transition scenario is discussed in detail including
a critical review of existing approaches and results for the ionic Hubbard
model.
