  Ab initio calculations face the challenge of describing a complex multiscale
quantum many-body system. The nuclear wave function has both strong short-range
correlations and long-range contributions.
  Natural orbitals provide a means of adapting the single-particle basis for ab
initio no-core configuration interaction (NCCI) calculations to better match
the many-body wave function. Natural orbitals are obtained by diagonalizing the
one-body density matrix from a calculation using an initial single-particle
reference basis, such as the traditional harmonic oscillator basis. The natural
orbital basis builds in contributions from high-lying oscillator shells, thus
accelerating convergence of wave functions, energies, and other observables.
  The convergence of the ground and excited state energies, radii, and
electromagnetic observables of He, Li, and Be isotopes calculated using natural
orbitals in ab initio NCCI calculations is discussed. It is found that
electromagnetic observables involving the M1 operator fully converge, while the
calculated energies, radii, and observables involving the E2 operator converge
significantly faster with the natural orbital basis than with the harmonic
oscillator basis. The use of infrared (IR) extrapolation schemes with the
natural orbital calculations is also explored.
