tensile loading at two ends using velocity command.

hi lammps users ,

I need your help. I am simulating a system of BN nanosheet using lammps and tersoff potential parameters. I want to apply tensile loading at two ends of the BN nanosheet using velocity set/create command. Can anybody suggest me how can I do this? currently I applied tensile loading using fix deform command but my results are not matching with the paper I am referring. In this paper velocity command has been used by author for tensile loading.I am getting higher values of failure strains. The original paper has failure at strains 0.23 and mine is coming out to be 0.30. Is this due to that I am using fix deform command? I am getting this results using both cases i.e. ppp and ppf boundary conditions.

thanks. hope for a positive and quick response.

If you initialize a group of atoms with a velocity and zero the

forces on the atoms via fix setforce, and integrated with fix nve,

then those atoms will continuously move with the intitial vel.

You can do that multiple times for multiple groups of atoms.

Steve

Thank you Steve. I tried using groups but box size didn’t change. Only atom moved from their position. So lx and ly remained constant. So strain values remained zero. I am using periodic boundary conditions.

Rajesh.

Moving atoms with fix nve doesn’t change the box size - why do

you think it should?

Steve

Thank you Steve. I tried using groups but box size didn’t change. Only atom moved from their position. So lx and ly remained constant. So strain values remained zero. I am using periodic boundary conditions. In another instant when I used s p p conditions box size changed but not at regular steps. It changes after a large number of steps and value showed a jump from .000000102 to 0.0803. How can I change it at every step?

If moving atoms doesn’t change size then how can I calculate strain?

Thank you Steve. I tried using groups but box size didn’t change. Only atom moved from their position. So lx and ly remained constant. So strain values remained zero. I am using periodic boundary conditions. In another instant when I used s p p conditions box size changed but not at regular steps. It changes after a large number of steps and value showed a jump from .000000102 to 0.0803. How can I change it at every step?

You don’t need that information. Since you know the size if your system at the start and the (constant!) velocity of your pulling atoms, you can easily infer the dimensions from that.

Please note, that using a periodic system with fix deform and this pulling approach are simulating somewhat different systems that only become equivalent in the limit of very small velocities.

Axel

Thank you Axel for your response. But I need strain values at every step so that I can plot stress-strain graph. And other thing do you mean that fix deform command should be used with ppp conditions?

Can I use fix deform and velocity commands interchangeably?

Thank you Axel for your response. But I need strain values at every step so that I can plot stress-strain graph.

And I explained in my previous email, how you can keep track of it. It is really straightforward. You don’t need box dimensions. You can enforce to update shrink-wrap boundaries by forcing neighbor list updates in every step

And other thing do you mean that fix deform command should be used with ppp conditions?

No.

Can I use fix deform and velocity commands interchangeably?

No.

Axel