Electronic structure maximum symmetry analysis

As remarked above, Mulliken populations ( gross or net ) commonly fail to satisfy the physical constraints (4a, b). The natural orbitals (NOs) 0 of equation (I) are intrinsically best possible for condensing maximum occupancy in the smallest possible number of orbitals. However, they are rather unsuitable for chemical analysis, since they transform as irreducible representations of the full symmetry group of F (the symmetry of the Hamiltonian) and thus appear delocalized over all nuclear centers and highly non-transferable. In this respect, NOs are similar to canonical molecular orbitals (to which they reduce in the uncorrelated Hartree-Fock limit). In contrast, the NBOs retain the high occupancy of NOs, but have local one-and two-center character that takes advantage of the high transferability of localized electron pairs of the molecular Lewis structure. 

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