Adsorbate Modeling

Hi,

I want to do CH4 adsorption simulation using Sorption module. My question is how can I model CH4 molecules? I want to use Trappe model considering united atom approach. I cheked the previous discussions and could not find an answer about this issue. I can only select one force field. Does this force field estimate the potentials for both CH4 and MOF?

Thanks for your help.