Ab initio calculations of nuclei face the chal-lenge of simultaneously describing the strong short-range inteucleon 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.