The Natural Bond Orbital (NBO) program NBO 6.0 is a discovery tool for determining chemical bonding descriptors of complex wavefunctions. NBO 6.0 is the current implementation of the broad range of 'natural' algorithms for optimally expressing numerical solutions of the quantum mechanical Schrödinger equation in the chemically intuitive language of Lewis-like bonding patterns and associated resonance-type 'donor-acceptor' interactions.

Through their close association with elementary Lewis structure diagrams, NBOs provide a direct link to familiar valency and bonding concepts. Unlike delocalized MOs (whose sprawling forms vary bewilderingly even between closely related systems), NBOs are highly conserved and transferable from one molecular environment to another.

NBOs are intrinsic realizations of orbital-type bonding concepts as originally envisioned by Pauling, Mulliken, and Coulson. Unlike methods based on derivatives of charge density (a quasi-classical concept), NBO analysis exhibits the unifying beauty of quantum phase-matching and superposition principles in all chemical phenomena.

NBO provides mutually consistent and comprehensive analysis tools, ensuring harmonious chemical interpretations from one property to another. The program is uniformly implemented in leading electronic structure packages, providing an authoritative framework for state-of-the-art professional discourse. Widespread acceptance of the NBO paradigm is reflected in the burgeoning number of published applications (currently, ~1500 per year).