  The one-electron spectral-weight function of the half-filled ionic Hubbard
model is calculated by means of Quantum Monte Carlo. A metallic regimen occurs
between two values of the coupling constant (Hubbard U) U1 < U2 . The system is
a band insulator below U1, and an increasingly antiferromagnetic (AF) insulator
above U2 evolving into a Mott insulator as U tends to infinity . The
intermediate regimen which is both AF and dimerized, is caused by a two-peak
structure at k = 0,5 pi (the Fermi surface). As U increases both peaks approach
each other, overlap, and separate again, the system becoming metallic in the
overlap region.
