Accurate multideterminant ground-state energies of circular quantum dots containing N≤13 electrons as a function of interaction strength have been evaluated by the diffusion quantum Monte Carlo method. Two unique features are found for these confined two-dimensional systems: (1) as the electron density decreases, the quantum dots favor states with zero orbital angular momentum (L=0); and (2) for some values of N, the ground state cannot be fully spin-polarized because of a symmetry constraint. © 2009 The American Physical Society.