  We investigate the ground state phase diagram of the half-filled repulsive
Hubbard model in two dimensions in the presence of a staggered potential
$\Delta$, the so-called ionic Hubbard model, using cluster dynamical mean field
theory. We find that for large Coulomb repulsion, $U\gg \Delta$, the system is
a Mott insulator (MI). For weak to intermediate values of $\Delta$, on
decreasing $U$, the Mott gap closes at a critical value $U_{c1}(\Delta)$ beyond
which a correlated insulating phase with possible bond order (BO) is found.
Further, this phase undergoes a first-order transition to a band insulator (BI)
at $U_{c2}(\Delta)$ with a finite charge gap at the transition. For large
$\Delta$, there is a direct first-order transition from a MI to a BI with a
single metallic point at the phase boundary.
