Using NEB method with gas-phase species

[arosen]

How does one use the NEB method to find a transition state where the reactants or products consist of an isolated gas-phase species? For instance, I'm looking at a paper where isobutane reacts with a surface H on HZSM-5, resulting in gas-phase H2 and a surface-bound isobutyl radical. It's unclear to me how to use the NEB method in a case like this since the product's potential energy consists of two infinitely separated species. Is my only option to use a min-mode following method like the dimer method?

[graeme]

You can start with the molecule above the surface by about 5 Ang and then let the system relax. If there is a barrier to adsorption, it will relax to a minimum above the surface (in the gas phase). Then your final state can be with the molecule adsorbed on the surface, and you can do a regular NEB calculation to find the adsorption barrier.