The Natural Bond Orbital Analysis of Weinhold and coworkers (J. Chem. Phys. 83 (1985) 735, J. Chem. Phys. 83 (1985) 1736 and J. Chem. Phys. 78 (1983) 4066) can be called by the use of the NBO card. It reads from a density or orbital record, and performs the necessary transformations to Natural Atomic Orbitals (NAO), Natural Bond Orbitals (NBO) and Natural Localized Molecular Orbitals (NLMO). The latter can also be saved to a record and later used in local correlation treatments (cf. Section 32). By default, the full orbital space is used. The core orbitals can, however, be left out of the procedure if core_option=0.
One can choose to truncate the transformation series (e.g., only compute the NAO orbitals), with help of the LEVEL keyword. If level=1, only the NAO transformation will be carried out. For level=2 the NBO transformation is performed, and for 3 the NLMO (default).
Sometimes, the NBO procedure will not converge due to a bad ordering on the 2-center bond search. The first run is based on the Wiberg bond index, but the algorithm switches to the atom ordering on the subsequent runs. This can be avoided by the use of the option KEEP_WBI. If wbi_option=1, the Wiberg bond index is used in all iterations.