Zero force with fix store/force command

Hi All,

I am trying to find the critical resolved shear stress in LAMMPS.For that, i am planning to apply shear through “”"“velocity”""" command WITHOUT RAMPING…(To Simulate Dislocation Motion). For beginning, i am first doing the same in a perfect crystal containing only 500 atoms( IF SHEAR IS APPLIED IN CORRECT WAY IN A PERFECT CRYSTAL WITH DEFAULT ORIENTATIONS, THEN I WILL PROCEED TO THE ORIGINAL PROBLEM). I am doing on Aluminium, a=4.05A and using Al_jnp.eam potential.I am dumping the shear force using fix store/force command…
But log file gives the force as ZERO in the upper region… I am not able to figure as to where am i making mistake. I am attaching the i/p file of Shear…

##################SHEAR########################

fix 10 boundary nve
fix 11 boundary temp/rescale 10 300.0 300.0 10 1.0
fix 12 mobile nvt temp 300.0 300.0 .01
fix 14 lower setforce 0.0 0.0 0.0

velocity upper set .00000018075 0 0 ###########Box height is 18.075A, so this gives strain = 1e-5 after running for 1000 steps.

fix 13 upper store/force
compute stress upper reduce sum f_13[1]

variable strain equal 1e-5

variable shearforce equal (c_stress/(lxlz)) ######## Output from compute is in ev/A units, so this gives in ev/A^3 units…
variable shearstress equal (v_shear
160.2) ############ Stress in GPa

dump 1 all cfg 100 dump.* id type xs ys zs
thermo_style custom step temp pe v_strain v_shearstress

run 1000

minimize 1e-5 1e-5 10000 10000

Hi All,

I am trying to find the critical resolved shear stress in LAMMPS.For that, i
am planning to apply shear through """"velocity"""" command WITHOUT
RAMPING...(To Simulate Dislocation Motion). For beginning, i am first doing
the same in a perfect crystal containing only 500 atoms( IF SHEAR IS
APPLIED IN CORRECT WAY IN A PERFECT CRYSTAL WITH DEFAULT ORIENTATIONS, THEN
I WILL PROCEED TO THE ORIGINAL PROBLEM). I am doing on Aluminium, a=4.05A
and using Al_jnp.eam potential.I am dumping the shear force using fix
store/force command....
But log file gives the force as ZERO in the upper region... I am not able
to figure as to where am i making mistake. I am attaching the i/p file of
Shear....

your mistake is that you didn't read the documentation of fix
store/force to the very end.
it explicitly says that this fix is not invoked during minimization.

in addition, i don't see the point of using fix store force for your purpose;
you can just output the force or its components in a custom style dump
right away.

axel.

There are other neater ways to calculate the RSS. Why not do a simple uniaxial deformation using fix deform and multily the stress with the schmidt factor for a {111} type of plane for a FCC crystal.