The Lewis-type NBOs determine the localized Natural Lewis Structure (NLS) representation of the wavefunction, while the remaining "non-Lewis"-type NBOs complete the span of the basis and describe the residual "delocalization effects" (i.e., departures from a single localized Lewis structure). Thus, NBOs provide a valence bond-type description of the wavefunction, closely linked to classical Lewis structure concepts, "filled" (donor) Lewis-type NBOs and "empty" (acceptor) non-Lewis NBOs.