I want to model a reaction catalysed by periclase ($\ce{MgO}$) using DFT. I have a good guess on the transition state (TS) of the reaction that goes in gas phase/solvent (produced using MOPAC).
The proposal for this reaction goes by the Eley–Rideal mechanism and I have a good guess on how the reactant adsorbs.
Now comes the question: I would like to optimise the TS on the surface using DFT (either ORCA or NWChem), but in tractable time.
I am tempted to use a simple, fixed model for the surface, e.g., a set of fixed atoms in bulk arrangement. Can this work? How am I supposed to find a structure with just one imaginary frequency using a partially fixed model (maybe using some clever vibrational projections)?
Even though NEB could be applied, the implementation of it in NWChem does not work with fixed atomic coordinates. Besides, I am confident on the TS I found without the catalyst, so I would like to use that instead of starting all over again.
Edit: what about using a very small cluster model, say, 8-16 atoms, and let it relax? Do you think it could work (in the sense of giving a reliable TS in terms of geometry and energy)?