Natural Bond Orbitals

Here is the center for a much more sophisticated treatment of Lewis' bonding model with bonding and lone pairs, the octet rule and much more. Frank Weinhold and colleagues have developed this great computational tool since nearly 30 years at the Department of Chemistry, University of Wisconsin. On the link to the homepage, above, you'll find excellent tutorial material. The NBO program can be called within the Gaussian QC package. For Gamess and other QC programs you have to install an NBO plugin, available at the above link for a nominal fee.

What can NBO do ?

Many chemists are avoiding QC programs although these are now profusely available for PC's and Mac's or on Linux computers. Assuming satisfactory computer literacy, the reason is that they have not learned to interprete the results of MO computations with so called canonical orbitals. These are delocalized over the whole molecular skeleton and do not reflect the concept of localized electron pair bonds of G.N. Lewis with which all chemists have been educated. There exist several schemes (mentioned elsewhere) for "localizing" transforma-tions of canonical orbitals and thus helping chemists to identify their familiar bonding patterns. The NBO concept and its several subconcepts are by far the most advanced developments in this research field. Are they necessary to understand chemistry?

For my own reflections on chemistry I have not used NBO schemes extensively. Aside from some familiarity with the usual canonical orbitals - having done a lot of molecular spectroscopy! - I feel quite happy with traditional MO theory. The resulting wavefunctions can be linearly combined in a multitude of ways for many different purposes of "better understanding".
I find the perusal of spherical atomic orbitals (as orthonormal sets) for various combinations, mimicking Lewis structures with 3-digit precision, not really of much educational value. The learning curve (for me!) is steeper than to understand and use canonical orbitals! It may be different for you and I want to encourage you to try NBO if you are not satisfied with qualitative Lewis, quantitative Kimball or professional Mulliken, and Kohn-Sham. In any case, IMHO NBO is not suitable for chemical novices, i.e. at highschools and beginning university level.