The question about command "setforce" and "pyy" output

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1

Dear lammps user,

Recently I made a tensile test simulation of a polymer bar model. I used two methods to get the stress at different strain.

First, I used the command “setforce” to fix the two ends of the bar and assigned a velocity to one end. After dumping the output of “setforce,” I divided them by the area of the bar.

Second, I used the command “compute stress/atom”. The code is following:

compute		peratom all stress/atom NULL
compute		p all reduce sum c_peratom[1] c_peratom[2] c_peratom[3]
variable	press equal -(c_p[1]+c_p[2]+c_p[3])/(3*vol)
thermo_style	custom step temp etotal press v_press 

The problem is these two kinds of stress were not at the same magnitude. I don't know whether the problem is my code. 



Has anyone met such a problem? If the code is right, should these two methods get the same stress?



Best

Chi
2

These two methods would of course give you different answers. One is stress from the ends only the other is all the atoms; one is normalized by the area the other is by the volume.

To get a stress-strain curve, you should use only forces from the free atoms (without ends). It is your choice whether to normalize it by the cross sectional area or not.

Ray

3

Thanks Ray. I think you are right and appreciate your advice.

Best
Chi