  We report quantum Monte Carlo calculations of ground and low-lying excited
states for nuclei with A \leq 7 using a realistic Hamiltonian containing the
Argonne v18 two-nucleon and Urbana IX three-nucleon potentials. A detailed
description of the Green's function Monte Carlo algorithm for systems with
state-dependent potentials is given and a number of tests of its convergence
and accuracy are performed. We find that the Hamiltonian being used results in
ground states of both 6Li and 7Li that are stable against breakup into
subclusters, but somewhat underbound compared to experiment. We also have
results for 6He, 7He, and their isobaric analogs. The known excitation spectra
of all these nuclei are reproduced reasonably well and we predict a number of
excited states in 6He and 7He. We also present spin-polarized one-body and
several different two-body density distributions. These are the first
microscopic calculations that directly produce nuclear shell structure from
realistic interactions that fit NN scattering data.
