Term: natural bond orbital (NBO)
https://doi.org/10.1351/goldbook.NT07077

Definition:
The orbital which is formed from natural hybrid orbitals. For a localized σ-bond between atoms A and B, the NBO is: \[\sigma _{\text{AB}}=c_{\text{A}}\ \text{h}_{\text{A}}+c_{\text{B}}\ \mathrm{h}_{\text{B}}\] where hA and hB are the natural hybrids centred on atoms A and B. NBOs closely correspond to the picture of localized bonds and lone pairs as basic units of molecular structure, so that is possible to conveniently interpret ab initio wave-functions in terms of the classical Lewis structure concepts by transforming these functions to NBO form.

Source: PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1954 (https://doi.org/10.1351/pac199971101919)

Citation: 'natural bond orbital (NBO)' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.NT07077

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