Question about velocity set in Tension Simulation


Recently, I was trying to simulate a tensile test of a laminate material containing SiC and graphene layer embedded. I make the atoms on one side of the model to be immobilized and use velocity set to the atoms on the other side to stretch the material. I read the manual that velocity set can give the group of atoms velocities in three directions respectively. However, I used velocity set 0.01 0 0 units box and the model can hardly ever be stretched. From the visualization I cannot see obvious change at the pulling direction. When I increased the velocity to 0.05, now I can observe the model being stretched, but this only lasted for a while, although this time the stretch is very obvious. So, I feel like I have some misunderstanding on the velocity set. It seems to me that I cannot pull a material with this command using relatively small velocity if the material is hard to be stretched. Could somebody give me some clues about this phenomenon? (Please tell me if I need to provide the input file and log file.)

Thank you

The “velocity set” command only sets an initial velocity. For as long as those atoms also experience a force, those velocities will changes. Just look at Newton’s laws of motion! So to retain the velocity on those atoms you also have to remove the forces.
At that point, you could just as well use fix move on those atoms instead of going through the hoops of assigning a velocity.

Thank you for the reply! Oh, now I understand it. Yes, I think fix move will be better for me to use in this case. That’s really helpful. Thank you very much sir!