ONIOM-type methods

Figure 3 Schematics of the three-dimensional extrapolation scheme, IMOMO( G2 ). Use a G2-type method for extrapolation of correlation methods and basis sets, and the ONIOM scheme for extrapolation of the molecular size. The points represented by black circles are actually calculated values, and those represented by shaded circles are extrapolated values...

The dummy junction atom or link atom approach introduces so-called link atoms to satisfy the valence of the atoms on the QM side of the QM/MM interface. Usually this atom is a hydrogen, but other atom types have also been used, e.g. halogens such as fluorine or chlorine. The link atom method can be used with both the Warshel-type QM/MM methods and ONIOM methods. The link atom method has been criticised because it introduces extra unphysical atoms to the system, which come with associated extra degrees of freedom. Another problem is that a C-H bond is clearly not chemically exactly equivalent to a C-C bond. Despite these problems, the simplicity of the link atom method means that it is used widely in the QM/MM modelling of proteins and other biological molecules. ... 

Experiments showed that both ee and ea isomers are present in solution for xantphos-type ligands and Landis therefore studied a series of 56 isomers and conformers along the reaction pathway by similar ONIOM methods. 

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