  We consider a Hubbard model on a square lattice with an additional
interaction, $W$, which depends upon the square of a near-neighbor hopping. At
half-filling and a constant value of the Hubbard repulsion, increasing the
strength of the interaction $W$ drives the system from an antiferromagnetic
Mott insulator to a $d_{x^2 -y^2}$ superconductor. This conclusion is reached
on the basis of zero temperature quantum Monte Carlo simulations on lattice
sizes up to $16 \times 16$.
