Dear all,

I want to simulate a stretching CNT and calculate its tensile strength. I need to calibrate the system for more complex geometries. I’d like to reproduce the results in https://www.sciencedirect.com/science/article/pii/S2095034915000525.

I have produced a 10x10 armchair, 4.8 nm CNT using scikitnano and want to compare rebo vs airebo vs airebo-m for several cutoff parameters with LAMMPS.

I perform energy minimization, followed by a NVE + Berendsen relax and a NPT relax.

Finally I apply a “fix all deform 1 z scale 2”. (I also tested the erate option instead of scale for comparison).

I calculate the strain rate as tot_ratio/tot_time and the stress as follows:

variable dt equal 0.001 #ps

timestep ${dt} #ps

#---------------STRESS-------------------------------------

compute 1 all stress/atom NULL

compute 2 all reduce sum c_1[3]

fix 1 all ave/time 1 100 100 c_2

variable sigmazz equal f_1

##--------------- STRAIN -----------------------

variable Zratio equal 2

variable srate equal ${Zratio}/${Nruns}/${dt} #/ps

variable strain equal step*v_srate*v_dt

###---------------DEFORMATION--------------------------------------

fix 2 all ave/time 1 1 100 v_strain v_sigmazz file deform.dat

dump 1 all atom 100 dump_deform.lammpstrj

fix 3 all npt temp 300 300 0.1 x 0 0 0.5 y 0 0 0.5

fix 4 all deform 1 z scale ${Zratio}

I get at least three problems:

- airebo-m shows no improvement with respect to rebo: there is an un-physical hardening above 5% of strain
- there is almost no difference when changing the LJ cutoff from 2.0 to 5.0 Ang
- generally I’d expect a tensile strength one order of magnitude larger (about 100GPa).

I can imagine there is either a problem in my stress calculation or generally in my minimization/relaxation scheme. Could anyone suggest a solution or improvement?

Please find attached my input and geometry files for reproduction.

Thank you.

input.lammps (3.1 KB)

cnt_10.lmp (33.5 KB)