  We use a diffusion Monte Carlo method to calculate the lowest energy state of
a uniform gas of bosons interacting through different model potentials, both
strictly repulsive and with an attractive well. We explicitly verify that at
low density the energy per particle follows a universal behavior fixed by the
gas parameter na^3. In the regime of densities typical for experiments in
trapped Bose-condensed gases, the corrections to the mean-field energies
greatly exceed the differences due to the details of the potential.
