page_title: Natural bond orbital
section_title: Lewis structures
hierarchical_section_title: Natural bond orbital / Lewis structures
caption_attribution_description: English: Amide resonance structures
context_page_description: In quantum chemistry, a natural bond orbital or NBO is a calculated bonding orbital with maximum electron density. The NBOs are one of a sequence of natural localized orbital sets that include "natural atomic orbitals", "natural hybrid orbitals", "natural bonding orbitals" and "natural localized molecular orbitals". These natural localized sets are intermediate between basis atomic orbitals and molecular orbitals:
  Atomic orbital → NAO → NHO → NBO → NLMO → Molecular orbital
  Natural orbitals are used in computational chemistry to calculate the distribution of electron density in atoms and in bonds between atoms. They have the "maximum-occupancy character" in localized 1-center and 2-center regions of the molecule. Natural bond orbitals include the highest possible percentage of the electron density, ideally close to 2.000, providing the most accurate possible “natural Lewis structure” of ψ. A high percentage of electron density, often found to be >99% for common organic molecules, correspond with an accurate natural Lewis structure.
context_section_description: With a computer program that can calculate NBOs, optimal Lewis structures can be found. An optimal Lewis structure can be defined as that one with the maximum amount of electronic charge in Lewis orbitals (Lewis charge). A low amount of electronic charge in Lewis orbitals indicates strong effects of electron delocalization.
  In resonance structures, major and minor contributing structures may exist. For amides, for example, NBO calculations show that the structure with a carbonyl double bond is the dominant Lewis structure. However, in NBO calculations, "covalent-ionic resonance" is not needed due to the inclusion of bond-polarity effects in the resonance structures. This is similar to other modern valence bond theory methods.
caption: resonance structure of amide
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