Diffuse functions in natural bond orbital analysis.
We show that diffuse function augmentation of Pople basis sets at the 6-311G RHF and B3LYP levels strongly impact conclusions drawn from natural bond orbital (NBO) analysis. The large spatial extent of high quantum number Rydberg orbitals introduced by augmentation contribute importantly to the valence space of neighboring atoms due to the likely inadequacy of the 311 valence functions. In contrast, lesser anomalies are found for augmentation of double zeta type 6-31G. The energetic anomalies found for bond and antibond NBO descriptions, made nonlocal by augmentation, are most serious for molecules with four or more heavy atoms. For these cases augmentation can lead to nonphysical results. NBO results using Dunning-type correlation consistent orbitals exhibit much weaker basis set dependence than those using the Pople basis sets.