The ONIOM is a computational approach developed by Morokuma and co-workers. ONIOM is the hybrid method that enables different levels of theory to be applied to different parts of a molecule/system and combined to produce reliable geometry and energy at reduced computational time.

ONIOM computational approach was found particularly useful for biomolecular systems as well as for transition metal complexes and catalysis.

See also

• QM/MM
  QM/MM
  The hybrid QM/MM approach is a molecular simulation method that combines the strength of both QM and MM calculations, thus allowing for the study of chemical processes in solution and in proteins. The QM/MM approach was introduced in the 1976 paper of Warshel and Levitt .An important advantage...
  
  
• Steric effects
  Steric effects
  Steric effects arise from the fact that each atom within a molecule occupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds , and this may affect the molecule's preferred shape and reactivity.-Steric...
  
  (ONIOM to separate Steric effects vs. electronic effects)