Localized orbitals are an obvious option when the property of interest is localized. Core IEs is an example of such local properties. Another option is to use correlated natural transition orbitals (CNTOs) 69 , which use information from the excitation amplitudes of a lower-level model to generate an active space for the MLCC calculation, similarly to other approaches using virtual orbital spaces truncated on the basis of lower-level natural orbitals .