What you need to do in my opinion is:
velocity yourgroup set 0.0 0.0 0.0
fix W yourgroup setforce NULL 0.0 0.0 (Null should be set to the direction you want the atom goes)
What you need to do in my opinion is:
velocity yourgroup set 0.0 0.0 0.0
fix W yourgroup setforce NULL 0.0 0.0 (Null should be set to the direction you want the atom goes)