The qualitative relation of bond energies to bond orders and hybridization is among the most fundamental concepts of chemistry. It is therefore surprising that relatively few modern computational quantum chemistry methods explicitly use these concepts. For example, the natural bond order analysis (NBO) method of Weinhold transforms a variationally optimized quantum mechanical wavefunction into a localized form that corresponds to one-center lone pairs and two-center bonds that conveniently are related to chemical Lewis structures.