ONIOM method geometry optimization

What is the essence of the ONIOM method for geometry optimization of model clusters and why this method is convenient for calculations of large Si02 and Ge02 clusters ... 

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. 

T. Vreven, K. Morokuma, 6. Farkas, H. B. Schlegel and M. J. Frisch, Geometry optimization with QM/MM, ONIOM, and other combined methods. I. Microiterations and constraints, J. Comput. Chem., 24 (2003) 760-769. 

Svensson M, Humbel S, Froese RDJ, Matsubara T, Sieber S, Morokuma K (1996) ONIOM A Multi-Layered 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 + H2 Oxidative Addition, J Phys Chem 100 19357-19363... 

Table 12-3. Comparison of calculated structural parameters for the kaolinite(O)—H2O system obtained from geometry optimizations of a cluster model (ONIOM(B3LYP/SVP PM3 method) and the periodic DFT(PW91) approach) (static relaxation and MD simulation) [73], Bond lengths and interatomic distances are in A, angles are in degrees. Superscript w stands for water, subscripts distinguish O and H atoms of three different surface OH groups...

A similar extrapolation can be done for multi-level ONIOM models, although it requires several intermediate calculations. It should be noted that derivatives of the ONIOM model can be constructed straightforwardly from the corresponding derivative of the underlying methods, and it is thus possible to perform geometry optimizations and vibrational analysis using the ONIOM energy function. 

Table 2-2. 1-, 2- and 3-layered ONIOM calculations for the deprotonation energy (in kcal/mol) of NH —CnBuH — CO—NH—CH2 — CO—NH — CHnBu — COO- system using the optimized geometries by the respective methods (Reprinted with permission from Morokuma et al. [11]. Copyright 2006 American Chemical Society.)... 

Comparisons between optimized and X-ray structures were once again made by calculating root-mean-square (RMS) deviations. When comparing all heavy atoms in the protein, the total RMS deviation is approximately 1.7 A, irrespective of method for the model system or the ONIOM implementation (mechanical, ONIOM-ME, or electronic embedding, ONIOM-EE). The largest deviations occur for residues in the vicinity of the second monomer. Therefore, adding the second monomer to the model should improve the calculated geometries. 

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