  A method to calculate the one-body Green's function for ground states of
correlated electron materials is formulated by extending the variational Monte
Carlo method. We benchmark against the exact diagonalization (ED) for the one-
and two-dimensional Hubbard models of 16 site lattices, which proves high
accuracy of the method. The application of the method to larger-sized Hubbard
model on the square lattice correctly reproduces the Mott insulating behavior
at half filling and gap structures of $d$-wave superconducting state of the
hole doped Hubbard model in the ground state optimized by enforcing the charge
uniformity, evidencing a wide applicability to strongly correlated electron
systems. From the obtained $d$-wave superconducting gap of the charge uniform
state, we find that the gap amplitude at the antinodal point is several times
larger than the experimental value, when we employ a realistic parameter as a
model of the cuprate superconductors. The effective attractive interaction of
carriers in the $d$-wave superconducting state inferred for an optimized state
of the Hubbard model is as large as the order of the nearest-neighbor transfer,
which is far beyond the former expectation in the cuprates. We discuss the
nature of the superconducting state of the Hubbard model in terms of the
overestimate of the gap and the attractive interaction in comparison to the
cuprates.
