The pair density can computed through canonical virtual orbitals or PAOs. In their original work, Neese et al. constructed PNOs from canonical virtual orbitals, using amplitudes from a preceding MP2 calculation. In a later work, Riplinger et al. updated their methodology by computing PNOs using canonical PAOs, from semicanonical MP2 amplitudes. This is known as the domain-based local pair natural orbital (DLPNO) approach.