Dear LAMMPS users,
I’m doing research about PPTA (Kevlar) using LAMMPS (August 2019) and ReaxFF potential parameter sets developed by Liu et al. (field.reax.lg) and I want to do a tensile simulation of PPTA as a goal.
But I have a problem in imposing a temperature of PPTA which is necessary before the tensile simulation.
I can finish the minimization well, but when I start giving a temperature to PPTA, the structure of PPTA is messed and PPTA chains buckle showing the zig-zag shape.
I attached the OVITO image of PPTA structure before and after imposing a temperature here https://drive.google.com/open?id=1FrjkLCGyRQ1VsEpExtgHheT6gPEwboDC .
I also attached the input file, control file at the end of this mail.
I did many things to solve this messing structure problem.
For example,
1, I always use a simulation cell of size 3×5×2 PPTA unit cells (1680 atoms) to get a quicker reply, but I changed this to 6×10×4 PPTA unit cells (13440 atoms).
2, I once changed a time constant of pressure control as small as possible.
3, Since I didn’t add partial charge information to coordinate file at first, I added the partial charge information.
4, Since I didn’t use the control file at first, I am using the example control file which I got in LAMMPS.
5, I changed the LAMMPS version from August 2017 to August 2019.
6, I changed the way of imposing a temperature from NVT to NPT.
7, I deleted “velocity” command from input files because I got less structural messing problem.
8, I changed the way to control pressure from iso to aniso because I got less structural messing problem.
All of the above did not dramatically solve the structural messing problem.
I’m thinking that I might have problems in values in the control file.
I am also thinking that the pressure during simulation is high because the values of stress per atom are in order of 10^6 atmospheres.
If you notice any problem in my control file or input file please give me advice.
There are some previous studies about PPTA with LAMMPS and ReaxFF and potential parameter sets developed by Liu et al.
I am always referring them and it seems that they didn’t have the zig-zag shape after imposing a temperature. So, I am thinking that this is a problem.
Thank you very much. I look forward to hearing from you.
Best regards,
Naoki (from Japan)
-----------------------------------------------------------------INPUT FILE----------------------------------------------------------------------------------------------------------------------------------
##-----Setting-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
variable ts equal 0.1 #timestep
variable iT equal 500 #initial temperature
variable T equal 300 #target temperature
##-----Model setting------------------------------------------------------------------------------------------------------------------------------------------------------------------
units real
atom_style charge
boundary p p p
read_data ./model/PPTAmodel_chargesN.txt
replicate 3 5 2
region PPTA block INF INF INF INF INF INF units box
pair_style reax/c control.reax.1st lgvdw yes
pair_coeff * * ffield.reax.lg H C N O
fix 0 all qeq/reax 1 0.0 10.0 1.0e-6 reax/c #Cutoff distance is less than twice the smallest dimendion of the replicated simulation cell
timestep ${ts}
##-----Minimization-------------------------------------------------------------------------------------------------------------------------------------------------------------------
fix f1 all box/relax iso 0.0
minimize 1.0e-10 1.0e-12 10000 10000
unfix f1
reset_timestep 0
##-----Initial temperature heating 0K to 500K-------------------------------------------------------------------------------------------------------------------------------------
fix f2 all npt temp {iT} {iT} 100 aniso 0.0 0.0 1000 tchain 3 pchain 3 # When iso, tchain pchain values are 1. When aniso, 3 (default).
run 20000
unfix f2
reset_timestep 0
##-----Initial temperature cooling 500K to 300K ----------------------------------------------------------------------------------------------------------------------------------
fix f3 all npt temp {T} {T} 100 aniso 0.0 0.0 1000 tchain 3 pchain 3 # When iso, tchain pchain values are 1. When aniso, 3 (default).
run 50000
unfix f3
reset_timestep 0
##-----Temperature control 300K to 300K ------------------------------------------------------------------------------------------------------------------------------------------
fix f4 all npt temp {T} {T} 100 aniso 0.0 0.0 1000 tchain 3 pchain 3 # When iso, tchain pchain values are 1. When aniso, 3 (default).
run 100000
unfix f4
reset_timestep 0