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.