[en] We report quantum Monte Carlo calculations of ground and low-lying excited states for A=8 nuclei using a realistic Hamiltonian containing the Argonne v₁₈ two-nucleon and Urbana IX three-nucleon potentials. The calculations begin with correlated eight-body wave functions that have a filled α-like core and four p-shell nucleons LS coupled to the appropriate (J^π;T) quantum numbers for the state of interest. After optimization, these variational wave functions are used as input to a Green's function Monte Carlo calculation made with a new constrained path algorithm. We find that the Hamiltonian produces a ⁸Be ground state that is within 2 MeV of the experimental resonance, but the other eight-body energies are progressively worse as the neutron-proton asymmetry increases. The ⁸Li ground state is stable against breakup into subclusters, but the ⁸He ground state is not. The excited state spectra are in fair agreement with experiment, with both the single-particle behavior of ⁸He and ⁸Li and the collective rotational behavior of ⁸Be being reproduced. We also examine energy differences in the T=1,2 isomultiplets and isospin-mixing matrix elements in the excited states of ⁸Be. Finally, we present densities, momentum distributions, and studies of the intrinsic shapes of these nuclei, with ⁸Be exhibiting a definite 2α cluster structure. (c) 2000 The American Physical Society