Problems when using fix deform to get Young's and shear moduli

Dear Lammps users,

I am using fix deform command to obtain Young’s modulus and shear modulus. When I run the the following script, I encountered two problems:

  1. I can get reasonable Young’s modulus when the stretching strain is small (<5%). The tensile stress (uniaxial) of graphene is expected to abruptly drop down when the strain reaches around 20%. But my obtained stress keeps increasing. It seems that the graphene can not be broken down even the strain is already large enough (>30%).

  2. When I use “fix deform” to tilt the box, the shear stress in xy direction (pxy if I understand correctly) fluctuates and does not increase with the shear strain.

I searched related topics in the threads, but did not find any solution. Could anyone pointed out the problems/errors in my script and give me some advices?

units metal
dimension 3
boundary p p p
atom_style atomic
newton on

read_data data.graphene

pair_style airebo 2 1 0
pair_coeff * * CH.airebo C

timestep 0.0005
fix 1 all npt temp 300 300 0.5 y 0 0 0.5
thermo 20

compute 1 all stress/atom NULL
compute 2 all pe/atom
compute 3 all reduce sum c_1[1] c_1[2] c_1[3]
thermo_style custom step temp pe ke etotal lx ly xy pxx pyy pxy

run 30000

unfix 1
fix 1 all nvt temp 300 300 50

fix 2 all ave/atom 1 1000 1000 c_1[1] c_1[2] c_1[3] c_2 fx fy fz
dump 1 all atom 1000 dump.lammpstrj

fix 3 all deform 1 y erate 0.001 units box remap x (tensile strain, for Young’s Modulus)
fix 3 all deform 1 xy erate 0.001 units box remap x (shear strain, for Shear Modulus)

run 600000

Thanks in advance for your time and assistance.

I suggest you look in the literature to see if papers have done

this for the strain rates and sample size you are using. I would

also try it with fix nve or fix langevin before using fix nvt.

Steve

Dear Lammps users,

I am using fix deform command to obtain Young's modulus and shear modulus.
When I run the the following script, I encountered two problems:

1. I can get reasonable Young's modulus when the stretching strain is
small (<5%). The tensile stress (uniaxial) of graphene is expected to
abruptly drop down when the strain reaches around 20%. But my obtained
stress keeps increasing. It seems that the graphene can not be broken down
even the strain is already large enough (>30%).

2. When I use "fix deform" to tilt the box, the shear stress in xy
direction (pxy if I understand correctly) fluctuates and does not increase
with the shear strain.

I searched related topics in the threads, but did not find any solution.
Could anyone pointed out the problems/errors in my script and give me some
advices?

units metal
dimension 3
boundary p p p
atom_style atomic
newton on

read_data data.graphene

pair_style airebo 2 1 0

Is there a particular reason why you chose "2 1 0"? By doing so, you are
not using the published AIREBO potential.

Ray