  We have studied the ground state of the two-dimensional (2D) Hubbard model by
using a quantum monte method paying special attention to the shell structure
effect on finite size clusters. Our calculations show there is a gap for spin
excitations in the ground state and incommensurate peaks at $(\pi \pm \delta,
\pi)$ and $(\pi, \pi \pm \delta)$ in the spin correlation function for a low
lying excited state. In the ground state, the long range part of the d-wave
superconducting correlation function is enhanced and the momentum distribution
function at the Fermi level $(\pi,0)$ is rounded. The gap in spin excitations
and the momentum distribution function rounding is consistent with opening a
d-wave superconducting gap in the ground state.
