  The Coulomb gap in a donor-acceptor model with finite charge transfer energy
$\Delta$ describing the electronic system on the dielectric side of the
metal-insulator transition is investigated by means of computer simulations on
two- and three-dimensional finite samples with a random distribution of equal
amounts of donor and acceptor sites. Rigorous relations reflecting the symmetry
of the model presented with respect to the exchange of donors and acceptors are
derived. In the immediate neighborhood of the Fermi energy $\mu$ the the
density of one-electron excitations $g(\epsilon)$ is determined solely by
finite size effects and $g(\epsilon)$ further away from $\mu$ is described by
an asymmetric power law with a non-universal exponent, depending on the
parameter $\Delta$.
