I was curious to find the simple explanation about this word: ONIOM. You can believe me that (until I wrote this bog), the word ONION was not appeared in wikipedia. It comes in google but I beg you with already ‘advanced’ applications or uses. Sometimes, only some papers come with that word.
The basic meaning of ONIOM is computing the molecule with different level of theory simulataneously. The reason could be very simple, when you have a large molecule, it will spend a lot of time to calculate all parts with very precise method although you will get the ‘almost’ correct value. On the other hand, using low basis sets, you will produce untrustable result. So, in the middle, you can manage the important part of molecule, doing ‘more’ in term of calculation, and use the low level theory for other parts which don’t need to be very correct. As nowadays there are many basis sets for specific cases, the choice of that is the key of the correctness of the calculation. It requires many literature reviews before doing this case.
In ONION, you can split your molecule into three different levels of QC/MM or QC/QC. QC stands for quantum chemical while MM stands for molecular mechanical. You can find this definition in this blog or by googling I hope you will know it. The definition of these three levels of calculation will be mentioned after you defind the geometry of single atom in the input file of Gaussian. Most methods works with Oniom, e.g. optimisations and frequency calculations. However, SCRF does not work (it is not available for semiempirical or molecular mechanical methods). SCRF or self-consistent reaction field is usually used for the calcilation with solvation (solvent existence) or non gaseous-phase.
The example of ONIOM input in the Gaussian is:
#P Oniom(B3LYP/6-31G*:PM3:Amber) Opt Freq
A three-layer oniom calculation of CH3-CH2-CHO
0 1
h 1.303312051 0.000000000 1.949234062 High
c 0.375760127 0.000000000 1.351034669 High
o -0.718742386 0.000000000 1.904192435 High
c 0.583631613 0.000000000 -0.143478503 Medium H
h 1.171836119 -0.887154331 -0.398970957 Medium
h 1.171836119 0.887154331 -0.398970957 Medium
c -0.720914899 0.000000000 -0.916455389 Low H
h -1.320117994 0.884637788 -0.676628009 Low
h -1.320117994 -0.884637788 -0.676628009 Low
h -0.526482757 0.000000000 -1.993329341 Low
