By definition, natural orbitals are state-specific, reflecting the state-specific nature of correlation. Consequently, exploiting natural orbitals within multi-state calculations is not straightforward. The difficulty of applying natural orbitals to excited states stems from the fact that a one-size-fits-all truncation of orbital space is likely to introduce an imbalance in the excited-states description. In some cases, one can even imagine a situation when the truncation based on correlation for one state would exclude the orbitals that are needed to describe the principal character of another state. For example, choosing orbitals based on their contributions to the ground-state correlation energy is likely to provide a reasonable virtual space for low-lying valence states (e.g., HOMO-LUMO excitations), however, would exclude orbitals needed for the description of Rydberg states because diffuse orbitals are not very important for ground-state correlation. Nevertheless, several ideas have been explored