Force component

I used fix smd and couple type. but I have some question

  1. Before using fix smd, I used movelinear and read the forces by fcm. In this case I used fix setforce 0 NULL NULL to fix atoms type 1 in the direction of pulling . what i think in smd couple there is no need for that. yes?

2)when I have a loop and want that after a loop finished a fix become unfix, How should I use it? for exmaple following style is true?

Loop a

fix 1
unfix 1
fix 2
unfix 2
next a