How to set walls for coordination or restrict molecule motion

[zc_belief]

Dear All:

I'm using MTD to simulate the dissociation reaction of organic molecules on TiO2-supported single atoms. One CV is set to the coordination number, and the other to the distance. When the molecule doesn't dissociate, the coordination number is 0, but the molecule moves over a large area. Is there a way to restrict the molecule's movement? I tried setting a wall, but I'm unsure if this wall is suitable for the coordination number. Could you please guide me on how to set it? Thanks!