I am trying to follow the code of tensile test of polyethylene (https://icme.hpc.msstate.edu/mediawiki/index.php/MD_PE_deformation) to do a tensile test of rubber. I made a polybutadiene single chain and put it in the simulation box. Now, after applying the deformation in x-direction, it deformed a little bit but then it starts to rotate. I have tried to fix the left wall of the simulation box with fix wall command but for applying fix wall, no fix npt and periodic boundary condition is accepted. Then I used boundary condition as f p p because I am fixing wall in x-direction and I removed all fix npt, and just use fix nve. Now it doesn't rotate but still not elongated that much. After some elongation it starts to vibrate at almost same position. I would highly appreciate if anybody please help me to solve this problem.