The NBO program performs the analysis of a many-electron molecular wavefunction in terms of localized electron-pair bonding units. The program carries out the determination of natural atomic orbitals (NAOs), natural hybrid orbitals (NHOs), natural bond orbitals (NBOs), and natural localized molecular orbitals (NLMOs), and uses these to perform natural population analysis (NPA), NBO energetic (deletions) analysis, and other tasks pertaining to localized analysis of wavefunction properties, including natural resonance theory (NRT) and natural chemical shielding (NCS) analysis. This section provides a brief introduction to NBO algorithms and nomenclature.