Keeping one species fixed in space in MD

I am curious, when we use molecular simulation to simulate two different species, is there any way to fix one species in space? (For example MD of adsorption. I want to fix the MOF in space and let the gas settle in the cavity by itself.) It should be interacting with other particles, but movement is restricted. I tried fix move linear 0 0 0, but it gives unrealistic temperature values.
And if somehow this is possible, how should we equilibrate the case? considering both gas and mof moving or keeping only the gas movable.
I am not sure if this creates a clear picture in the mind of the reader, but if you are able to understand this, I would love to have your input.

You can try using fix spring/self or fix smd.

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