  A trial wave function is proposed for studying the instability of the
two-dimensional Hubbard model with respect to d-wave superconductivity. Double
occupancy is reduced in a similar way as in previous variational studies, but
in addition our wave function both enhances the delocalization of holes and
induces a kinetic exchange between the electron spins. These refinements lead
to a large energy gain, while the pairing appears to be weakly affected by the
additional term in the variational wave function.
