Hi all
I am using LAMMPS to model tensile test of graphene.I try to model graphene sheet and thermodynamics settings similar to previous work. I am using periodic boundary condition in plane and shrink-wrapped boundary for out of plane boundary and AIREBO potential. I use npt ensemble for equilibration, after that I use fix deform to apply strain to simulation box.I applied deformation by utilizing different ensembles.I use fix nvt and npt as below:
fix 1 all npt temp 300 300 .01 y 0 0 10
fix 2 all deform 1000 x erate 1e-3 units box remap x
or
fix 1 all nvt temp 300 300 .01
fix 2 all deform 1000 x erate 1e-3 units box remap x
I try different Pdamp and timestep, but stress-strain curve of my simulation has lots of fluctuation.moreover, yield stress is more than previous article!
Do you have any suggestion to improve this curve?
any help would be appreciated.
Hi Sahar,
It seems that the value for Tdamp is too low.
In the LAMMPS manual, it says that if you use too low Tdamp value the fluctuation in temperature will be high, which in turn may affect your pressure (stress) values. I recommend you to work with the value of Tdamp. You can test your sample with the value of 100 for Tdamp.
baris.
Hi Sahar,
It seems that the value for Tdamp is too low.
In the LAMMPS manual, it says that if you use too low Tdamp value the fluctuation in temperature will be high, which in turn may affect your pressure (stress) values. I recommend you to work with the value of Tdamp. You can test your sample with the value of 100 for Tdamp.
baris.
Hi Sahar,
you can try to change the internal cutoff of the AIREBO potential (1.7 rcmin_CC below) to a value between 1.9 and 2.0 angstrom. In this way you can cure the unphysical increasing of the stress near the fracture. Indeed near the cutoff distance (starting value for the switching function that bring to zero the interaction, see AIREBO paper) you have an increasing of the force between the atoms. More details can be found in this paper Shenderova, O.A., Brenner, D.W., Omeltchenko, A., Su, X., Yang, L.H., 2000. Atomistic modeling of the fracture
of polycrystalline diamond. Phys. Rev. B 61, 3877–3888. URL: http://link.aps.org/doi/10.1103/PhysRevB.
A very interesting paper that explain the problem is this
Atomistic simulations of nanotube fracture T. Belytschko, S. P. Xiao, G. C. Schatz, and R. S. Ruoff Phys. Rev. B 65, 235430 – Published 20 June 2002 DOI:https://doi.org/10.1103/PhysRevB.65.235430
For an idea of the difference in using different cutoff values you can see this video https://www.youtube.com/watch?v=yoKSeoQEx6w, for the carbon nanotubes case.
This change was done by many author and is in general useful. For the choice of the cutoff value in the range 1.9 - 2.0 is important knowing what is the properties that on want compute.
Here the first part of the force field file
# DATE: 2011-10-25 CONTRIBUTOR: Ase Henry, [email protected] CITATION: Stuart, Tutein and Harrison, J Chem Phys, 112, 6472-6486 (2000)
# AIREBO Brenner/Stuart potential
# Cite as S. J. Stuart, A. B. Tutein, J. A. Harrison,
# "A reactive potential for hydrocarbons with intermolecular interactions",
# J. Chem. Phys. 112 (2000) 6472-6486.
1.7 rcmin_CC ## LINE IN WHICH TO CHANGE the value
1.3 rcmin_CH
1.1 rcmin_HH
2.0 rcmax_CC
1.8 rcmax_CH
1.7 rcmax_HH
...
..
.
Best,
Andrea
Dear Beris Demir
I really appreciate your answer. I have try different value of Tdamp and Pdamp in these days,but the answers have not changed a lot. finally,I've found these fluctuation appear due to wrong boundary selection. I used shrink boundary for out of plane boundary but I think I should use periodic or fix boundary.
Regards
Sahar
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