Integrated MO + MM method

W C, A Tempcz)rrk, R C Hawley and T Hendrickson 1990. Semianalytical Treatment of Solvation for Molecular Mechanics and Dynamics. Journal of the American Chemical Society 112 6127-6129. ensson M, S Humbel, R D J Froese, T Matsubara, S Sieber and K Morokuma 1996. ONIOM A Multilayered Integrated MO + MM Method for Geometry Optimisations and Single Point Energy Predictions. A Test for Diels-Alder Reactions and Pt(P(t-Bu)3)2 + H2 Oxidative Addition. Journal of Physical Chemistry 100 19357-19363. 

M. Svensson, S. Humbel, R. D. J. Froese, T. Matsubara, S. Sieber and K. Morokuma, 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)+H-2 oxidative addition, J. Phys. Chem., 100 (1996) 19357-19363. 

Three such methods have been proposed by Morokuma and coworkers. The integrated MO + MM (IMOMM) method combines an orbital-based technique with an MM technique. The integrated MO + MO method (IMOMO) integrates two different orbital-based techniques. The our own n-layered integrated MO and MM method (ONIOM) allows for three or more different techniques to be used in successive layers. The acronym ONIOM is often used to refer to all three of these methods since it is a generalization of the technique. 

One possible solution to this problem is a combination of the MO and the empirical molecular mechanics (MM), treating the active and important part of the molecule with the MO method and the remainder, such as bulky substituents or other chemical environments, with the MM method. Such treatments have been made for some organometallic systems [83-86]. However, in these treatments, only the geometry of the MM part is optimized under the assumption that the MO part is frozen at the optimized geometry of the small model system. This frozen assumption can result in a substantial overestimation of the MM energy. Recently, Maseras and Morokuma have proposed a new integrated MO + MM scheme, called IMOMM, in which both the MO part and the MM part of geometry are simultaneously optimized [87]. This method can combine any MO approximation with any molecular mechanics force field. The application of this method at the IMOMM(HF MM3) and IMOMM(MP2 MM3) levels to the oxidation addition reaction of H2 to Pt(PRa)2, where R = H, Me, r-Bu, and Ph [88], has shown a promise that more realistic models of elementary reactions and catalytic cycles may be studied in the near future with this method. 

The so-called ONIOM (our Own N-layered Integrated molecular Orbital and molecular Mechanics) method is a generalization of the previous versions in this approach called IMOMM (Integrated MO+MM) and IMOMO ... 

Matsubara T, Sieber S, Morokuma K (1996) A test of the new integrated MO + MM (IMOMM) method for the conformational energy of ethane and n-butane. Int J Quantum Chem 60 1101-1109... 

Chem., 100, 19357 (1996). ONIOM A Multilayered Integrated MO 4-MM Method for Geometry Optimizations and Single Point Energy Predictions. A Test for Diels-Alder Reactions and Pt(P(t-Bu)3)2 + H2 Oxidative Addition. 

Combining MO and MM methods so that the metal and immediate ligand sphere is described by MO methods and the outer, purely organic part of the ligand by the much less expensive MM technique is also possible. One way to integrate molecular orbital and molecular mechanics is IMOMM, which allows steric effects of bulky ligands to be successfully modeled, for example." ... 

AMI Combined Quantum Mechanical and Molecular Mechanical Potentials Combined Quantum Mechanics and Molecular Mechanics Approaches to Chemical and Biochemical Reactivity Hybrid Methods Hybrid Quantum Mechanical/Molecular Mechanical (QM/MM) Methods Linear Scaling Methods for Electronic Structure Calculations Mixed Quantum-Classical Methods MNDO MNDO/d Parameterization of Semiempirical MO Methods PM3 Quantum Mechanical/Molecular Mechanical (QM/MM) Coupled Potentials Quantum Mechanics/Molecular Mechanics (QM/MM) Semiempirical Methods Integrals and Scaling. 

This article reviews the onion-like multi-layered integration method, ONIOM (our own N-layered integrated molecular orbital and molecular mechanics) method, developed and mainly applied by the group of present authors, for accurate calculations of large molecular systems which cannot be achieved with the standard molecular orbital (MO) or molecular mechanics (MM) methods. 

However, integration of MO and MM methods is like grafting a bamboo to a tree, and is rather complicated, awkward, and arbitrary, as was found previously in many QM/MM implementations. The physical meanings of MO energies (total energies) and MM energies (steric energies) are quite different. Therefore, for instance, in the IMOMM approach ... 

Semi-empirical MO methods (such as MNDO, AMI, PM3) use the frozen core approximation. All the integrals (except the overlap integrals) related to the valence electrons are calculated approximately. Therefore, the coupling Hamiltonian Hqm/mm should be reformulated to conform to the frozen core approximation in the QM calculation. The additional integrals (eq.(22) and eq.(23)) are also calculated approximately. For details, the reader should consult the papers. [12,19,36]... 

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