ONIOM (High level:Medium level:Low level) is the hybrid method of Morokuma and co-workers that enables different levels of theory to be applied to different parts of a molecule/system and combined to produce a consistent energy expression. How will we decide that any particular method is good for particular part of molecule?
Dear Alam
The choice of method is depends upon the size of the part in the molecule that you are really interested. For example if want to optimize the active site of a protein, then treat the active site as higher level theory and treat the other parts that are insignificant as lower level. The logic is simple ..... The part you want treat higher level and remaining lower.....
I hope this information could be useful....
The interested region of your bulky molecule (generally up to 100 atoms) should be pointed as higher level and the neighboring atoms as medium level ad the rest of the molecule as lower level. Please not that you have to use a precise level of theory for your higher level depend on number of atoms in it.
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