"It is nice to know that the computer understands the problem, but I would like to understand it too." - Eugene Wigner

The Natural Bond Orbital (NBO) program NBO 6.0 is a discovery tool for chemical insights from complex wavefunctions. NBO 6.0 is the current version of the broad suite of 'natural' algorithms for optimally expressing numerical solutions of Schrödinger's wave 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).

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Quick Slideshow Intro to NBO & NBOView

What Are NBOs?

NBO Keywords & Sample Output

NBO Tutorials

What's New in NBO 6?

[Check here for latest bugfix and update announcements]

March 14, 2017: A new, partially G16-enabled interim NBO6 distribution (NBO 6.0.15, dated 14-Mar-2017), is now available for download. This new distribution remains fully compatible with G09 Revs. D, E, and partially compatible with G16, Rev. A, except for some notable issues with NBO energetic analysis ($DEL options) and proper analysis of correlated MP2 or CI densities, as detailed below:
(i) NBO $DEL options are severely restricted. For the time being, closed-shell NBO deletions require a non-standard route (as described in installation instructions) and open-shell deletions don't work at all.
(ii) Correlated MP2 or CI densities cannot be properly analyzed. The Gaussian "density=current" keyword is not correctly implemented, so that only the SCF density (rather than the properly correlated MP2 or CI density) is provided to the NBO program for analysis.
   It is expected that these remaining G16-incompatbilities will all be fixed when the first minor update to G16 is released, most likely in May 2017. Devoted G09/NBO6 users may wish to defer upgrades from G09 to G16 until this first minor update to G16 is released by Gaussian Inc..
   The 14-Mar-2017 distribution includes a number of other features of interest to general NBO users, whether or not an upgrade to G16 is contemplated:
  • Dynamic memory allocation is now supported, allowing one to exceed the former 2Gb address space of 32-bit applications (see NBO Program Manual, p. B-6).
  • The default NAO search algorithm has been modified to better preserve core-valence separation in rare cases where numerical near-degeneracies can lead to unphysical core-valence mixing. The numerical effects are generally negligible except where they become necessary. The "OLDNAO" keyword restores the legacy NAO algorithm for comparison purposes.
  • A formatting limit in PLOT (.31) filies has been revised to allow applications to larger molecules.
  • Linux x86 binaries for NBO 6.0 were formerly prepared with Ubuntu 12, which is nearing "end of service" status. Present linux binaries are now prepared with Ubuntu 16, which no longer supports 32-bit x86 applications. Users requiring 32-bit linux applicability should order source code NBO 6.0 that can be compiled for the platform of interest.
It is recommended that all current NBO 6.0 license holders (except 32-bit linux users) consider updating to this version.
January 23, 2017: Gaussian 16 Compatibility with NBO6?
Recent release of Gaussian 16 presents significant interfacing issues with current NBO 6.0. Due to changes in the G16 matrix element file and erroneous standard route for POP=NBO6Del options (only reparable in some future G16 revision) significant code-patches may be required to achieve even partial compatibility with G16, while also preserving full interactive compatibility with G09 Revs. D, E.
   We intend to announce a new NBO 6.0 distribution in the near future that allows an interim level of G16/NBO6 interfacing and includes possible work-arounds for some known incompatibilities. In the usual manner, current NBO 6.0 license holders may use their original download code to obtain the patched code from the NBO repository and explore the possibilities of interim G16/NBO6 interfacing for their applications of interest. Keep watching this space for an announcement of availability.
August 24, 2015: Organometallics cover "illustrates the dilemma created by applications of different methods, here NBO and EDA-NOCV, for the extraction of bonding information from ab initio computations - which method is better? Because the analyses represent different interpretations, rather than observables, there is no bottom-line criterion for such a judgment. We are reminded of the complementary contrariness of Tweedledee and Tweedledum in Lewis Carroll's Through the Looking Glass and their inability to provide Alice with directions on the best way out of the woods. The cover was conceived by Russ Hughes, and the artwork was created by Kate Nikles." [C. R. Landis, R. P. Hughes, and F. Weinhold, "Bonding Analysis of TM(cAAC)2 (TM = Cu, Ag, and Au) and the Importance of Reference State," Organometall. 34, 3442-3449 (2015)]
   January 15, 2015: In recognition of the 2nd anniversary of NBO 6.0 release, the legacy NBO 5.9 program and website is being decommissioned from active maintenance and support. Users requiring NBO 5.9 for compatibility with NBO6-incompatible or other older ESS versions may still order through the legacy NBO5 order form.
   September 22, 2014: C&E News cover story "Pushing the Limits of Chemical Bonding" features NBO-based characterization of anti-electrostatic H-bonding and other challenging cluster interactions.
   June 1, 2014: New ORCA v. 3.0.2 release now provides improved (correlated) densities for NBO6-level analysis of DLPNO-CCSD(T) and other advanced methods (more...)
   May 14, 2014: Q-Chem, Inc. announces release of Q-Chem v. 4.2 with NBO6 interface included (more...)
   October 1, 2013: The Frank Neese group of MPI-Mülheim announces release of NBO6-compatible Orca v. 3.0 (more...)
   August 16, 2013: Schrödinger Inc. announces release 2013-2 of Jaguar v. 8.1, with NBO6 included (more...)
  July 28, 2013: NBO6-level NBOPro v.6 now available (more...).
  July 27, 2013: Marcel Patek announces a set of utilities to assist $NBO input preparation ("Gennbo Helper") and Jmol-based orbital visualizations ("Jmol-NBO Visualization Helper") in NBO-based applications. Companion videos provide useful tutorial introduction to these utilities (more...)
May 8, 2013: J. Comp. Chem. cover feature: "NBO 6.0, the next generation in natural bond orbital methods,... includes link-free connectivity for interfaces to popular electronic structure programs, new methods of analysis, and numerous algorithmic enhancements. The cover image is made with the NBOView 2.0 program and shows the sigma-type, 3-center/2-electron bonding NBO of the cyclobutenyl cation that arises from the strong overlap of the vacant valence orbital at C1 with the C2-C3 bond. Automated multicenter bond searches are one of the many new features of the NBO program." [JCC 34, 1472 (2013)]
   May 6, 2013: NBO6-compatible Gaussian 09 Rev. D.01 now available, with "pop=npa6", "pop=nbo6read", and related keywords (more...)
   April 11, 2013: NBO6-compatible Molpro v. 2012.1 now available (more...)
   April 8, 2013: SCM announces release of ADF2013, with NBO6 included (more...)
   March 12, 2013: "Early View" of J. Comp. Chem. article "NBO 6.0: Natural Bond Orbital Analysis Program" (DOI: 10.1002/jcc.23266) now available (more...)
   February 4, 2013: NBO6 binaries for PC-Windows now available (more...)

   JANUARY 20, 2013: NBO6 ANNOUNCEMENT DAY (more...)
   December, 2012: GAMESS releases NBO6-compatible source code version (more...)

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