I am trying to do a tensile test of rubber by MD Simulation using LAMMPS. I made a polybutadiene single chain and put it in the simulation box. Now, after applying the deformation in x-direction using erate function, it deformed a little bit but after some time it starts to rotate without any elongation. 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 it still is not elongated that much. After some elongation it starts to vibrate at almost same position.

Another problem, when using fix/wall function, sometimes it shows "particle is inside or on the fix wall" but when I changed the values of coordinate of the face, epsilon, sigma, cutoff then it works sometimes. Basically I am not sure how to define these parameters. My simulation box size is -15.00 to +15.00 for all x,y, and z.