how to set one side of rectangular wire fix (no motion in any direction)?

I want to fix one side of a ractangular wire fix so that it does not move and the other is free to move with some external force. I cannot understand how to fix one side of the sample so that it cannot move even due to the action of applied force at the other end. also kindly guaid me how can i get values of tensile stress ans strain after each time step?

Best Regards;
Muhammad Imran

You can either not time integrate those atoms (don't
put them in the group used with fix nve). Or set
their force and velocity to 0.0. See the fix setforce