Hi,
I would like to know if it is possible to study bond dissociation energies with the oniom method provided in gaussian. Let's say the dissociation of ethane (although this system might be too small, but for demonstrating purposes it should be ok). So I would split the molecule in a C2H5 fragment which represents the high-layer and the H fragment which represents the low-layer. If we have now the dissociation:
C2H6 -> C2H5 + H
then the bond dissociation energy is
( E(C2H5) + E(H) ) - E(C2H6).
My question is now:
Can I set these layers like this and calculate the bde or may run in inconsistencies regarding the different layers on product and reactants?
I would expect such an algorithm to have huge problems due to parts of molecules being in different laers and provide unreliable results. It's OK, if the entire molecule with bond breaking is in the same, high layer - otherwise tou can't really use it.
Hi Michael,
It doesn't make sense... the ONIOM algorithm is based on these equation
E(oniom)= E(high layer on model system) + E(low layer on real system)-E(low layer on model system)
According with your proposal, your high layer is ethane molecule (assuming that you are saturating your model limits with hydrogens) and your real system is also ethane, so in principle you will obtain just the energy of ethane at the high layer method.
In addition, if you think in a bigger molecule, it also would be problematic, due to the different methods in the limits of the layer, that's is the most critical part in ONIOM calculations and I would not recommend to run specific properties at this limit to obtain accurate values
Hey,
thank you very much for your answers which confirm my suggestions.
Another question is how accurately you require the BDE? That can restrict (or extend) the choice of QM approach.
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