The ONIOM (short for 'Our own N-layered Integrated molecular Orbital and Molecular mechanics'[1]) method is a computational approach developed by Keiji Morokuma and co-workers. ONIOM is a hybrid method that enables different ab initio, semi-empirical, or molecular mechanics methods to be applied to different parts of a molecule/system in combination to produce reliable geometry and energy at reduced computational cost.[2][3][4]
^S. Dapprich; I. Komaromi; K.S. Byun; K. Morokuma & M.J. Frisch (1999). "A new ONIOM implementation in Gaussian98. Part I. The calculation of energies, gradients, vibrational frequencies and electric field derivatives". Journal of Molecular Structure: THEOCHEM. 461–462: 1–21. doi:10.1016/S0166-1280(98)00475-8.
^Svensson, Mats; Humbel, StéPhane; Froese, Robert D. J.; Matsubara, Toshiaki; Sieber, Stefan; Morokuma, Keiji (1996). "ONIOM: A Multilayered Integrated MO + MM Method for Geometry Optimizations and Single Point Energy Predictions. A Test for Diels−Alder Reactions and Pt(P(t-Bu)3)2+ H2Oxidative Addition". The Journal of Physical Chemistry. 100 (50): 19357. doi:10.1021/jp962071j.
^Ananikov, Valentine P.; Musaev, Djamaladdin G.; Morokuma, Keiji (2010). "Real size of ligands, reactants and catalysts: Studies of structure, reactivity and selectivity by ONIOM and other hybrid computational approaches☆". Journal of Molecular Catalysis A: Chemical. 324 (1–2): 104–119. doi:10.1016/j.molcata.2010.03.015.