# [lammps-users] shear with a constant force

Dear Lammps Users,

We are interested to look at the dislocation mobility. As the dislocation velocity is proportional to the resolved shear stress, we are interested to simulate the motion of a screw or edge dislocation under a shear stress.

We set up a simple calculation using the command fix setforce with the corresponding value of the force (I will paste the input file a little bit later), and for a given temperature we tested different values of applied stress.

We observed the following:

(i) The velocity was independant of the stress (which is completely not in agreement with the experiment)
(ii) after something like 30ps, like in graphene, the upper part of the block of atoms, went back while the bottom layer, continued to move.

So, my question is the following: Could you please confirm that with the input file that I am using, a force is well applied along the X direction, the mobile atoms are integrated using a Nose/Hoover thermostat, the fixed layers are integrated with a nve thermostat with a rescaling of the temperature so the velocity is not updated.

Also, could you confirm that the force that we are giving to the atoms is a force/atoms. For example, if we want to apply a shear stress \sigma, the force that we need to apply is:
F = \sigma * A / N where N is the number of atoms in the layer and A is the surface of the layer.
\sigma = 10MPa,
N = 2800
A = 222*111 angstrom^2
F = 10e6 * 222*111*1e-20/2800*6.2e8 == 0.00054 eV/angstrom

Sebastien

# Test of MEAM potential for Mg system

units metal
boundary p s p

atom_style atomic

pair_style eam/sp
pair_coeff * * speam

neighbor 4. bin
neigh_modify every 1 delay 0 check yes

#***** deinite the block of atoms
region lower1 block INF INF INF 0.5 INF INF units box
region lower2 block INF INF 0.5 2.7 INF INF units box
region upper1 block INF INF 98.8 99.3 INF INF units box
region upper2 block INF INF 101. INF INF INF units box
group lower2 region lower2
group lower1 region lower1
group upper2 region upper2
group upper1 region upper1
group boundarytmp1 union lower1 lower2
group boundarytmp2 union boundarytmp1 upper1
group boundary union boundarytmp2 upper2
group mobile subtract all boundary