Uniform stretching the system at high strain rate

Dear all
I need to simulate the tension test of a crystal to study the crack.
The boundary in Y and Z direction are periodic and in X direction is not
because of the nonperiodic condition in loading direction, I cant use
deform command.
I want to simulate loading at high strain rate (10^10 1/s) to save the
simulation time.
I need a uniform stretching of the system to avoid the creation of shock
wave during simulation and
I can't do this by moving the boundary (it will create shock)
please help me how I can do that.
Axel KohlmeyerJan 9 (4 days ago)
use fix move with an atom style variable for the displacement. axel. >
Steve Plimpton <[email protected]>
Jan 9 (4 days ago)
to Axel, me, LAMMPS
Or just give atoms on the free boundaries an initial
velocity and set the force on them to 0.0 via fix setforce.
They will then move at constant velocity.

Dear Steve
I can't understand the relation of set the force on boundary zero
and avoid of shock in simulation.
I have tried, but I couldn't do it OK.
please let me know more about it.

The velocity ramp command wiill give atoms an initial velocity
before starting a simulation. It can be used to give atoms a velocity
that matches the velocity they will have at time=0 when a large
box deformation is instantaneously applied, E.g 0 in the center
of the box, and some large velocity at the box boundary. Done
correctly, this avoids a large time=0 perturbation.