  The question if a Mott insulator and a band insulator are fundamentally
different has been the matter of intensive research recently. Here we consider
a simple model which allows by tuning one parameter to go continously from a
Mott insulator to band insulator. The model consists of two Hubbard systems
connected by single particle hopping. The Hubbard Hamiltonian is solved by the
Dynamical Mean-Field theory using Quantum Monte Carlo to solve the resulting
quantum impurity problem. The quasiparticle spectral function is calculated.
Here we focus on the optical conductivity and in particular on the Drude weight
which can be experimentally measured. From our calculation we conclude that
there is a continous crossover from the band insulator to the Mott insulator
phase at finite temperature.
