How to fix center of mass

Hello Everyone,

I am trying to form a water droplet on graphene sheet, to calculate its contact angle for better understanding. But, the droplet is moving all around the sheet. Can anyone tell me how to fix the center of mass of the droplet or how to get a droplet so that it can retain in one place?
20nm x 20nm graphene sheet with 3000 water molecules.
any command that can fix center of mass?