  Ab initio calculations of nuclei face the challenge of simultaneously
describing strong short-range internucleon correlations and the long-range
properties of weakly-bound halo nucleons. Natural orbitals, which diagonalize
the one-body density matrix, provide a basis which is better matched to the
physical structure of the many-body wave function. We demonstrate that the use
of natural orbitals significantly improves convergence for ab initio no-core
configuration interaction calculations of the neutron halo nucleus 6He,
relative to the traditional oscillator basis.
