  A momentum-space approach of the density-matrix renormalization-group (DMRG)
method is developed. Ground state energies of the Hubbard model are evaluated
using this method and compared with exact diagonalization as well as quantum
Monte-Carlo results. It is shown that the momentum-space DMRG is a very useful
numerical tool for studying the Hubbard model and other fundamental models of
interacting electrons in two dimensions. For the Hubbard model in two
dimensions, the momentum-space DMRG method reproduces accurately the exact
diagonalization results of ground state energies on a $4\times 4$ lattice and
yields new upper bounds of ground state energies on an 8$\times$8 lattice.
