[lammps-users] Shock Modelling of Polymer

Dear Lammps Users,

Currently I am working on modelling the shock wave for polymer material. I am using the LAMMPS 2017 version. I am modelling the polyethylene for the shock loading.

I am facing some problem those are as follows;

  • While applying the wall/piston command on the z lower dimension, the simulation box is moving along the shock direction and the box dimension increases constantly in the z direction.
    herewith i have attached the input script. kindly help me to find the problem in the manuscript.

VARIABLES

variable simname index polyethyene

##########Initializing simulation#################
units real
boundary p p p
atom_style charge

##Defining Atom Coordinates#####
#log log.${simname}.txt
read_data data.pe

Defining potential####################

pair_style reax/c lmp_control
pair_coeff * * ffield.reax.BN H C
fix spcl all qeq/reax 20 0.0 10.0 1e-6 reax/c #param.qeq
neighbor 2 bin
neigh_modify every 2 delay 0 check no
write_dump all custom EQ_*.dump id type mass x y z q

Setting (calculating extra properties)

compute mytemp all temp
compute peratom all pe/atom
compute pe all reduce ave c_peratom
compute ke all ke/atom
compute ke1 all reduce ave c_ke

compute strs all stress/atom NULL virial
compute pr all reduce sum c_strs[1] c_strs[2] c_strs[3]

min_style cg
minimize 1e-15 1e-15 10000 10000

velocity all create 300.0 1231141 dist gaussian rot yes

Currently I am working on modelling the shock wave for polymer material.
I am using the LAMMPS 2017 version. I am modelling the polyethylene for the
shock loading. I am facing some problem those are as follows;
While applying the wall/piston command on the z lower dimension, the
simulation box is moving along the shock direction and the box
dimension increases constantly in the z direction.

I haven't read your script as carefully as I probably should, but here
are a few quick comments:
1) I can't tell if you have any other walls in your system besides the
piston. Presumably you want at least 2 walls in order to squeeze
stuff between them.
2) In stage 2 and later, you are running your simulations at constant
pressure, using fix npt. This will cause the volume of your
simulation to change. If you don't want this to happen, use "fix nvt"
(or "fix nve" + "fix langevin") instead of NPT.

(I don't think it makes sense to apply a piston to a simulation under
NPT conditions. Fix npt contains a barostat which attempts to control
the pressure by changing the shape of the simulation box. I don't see
why you would want to use "fix npt" if you are also using a piston.
Either you intend the piston and wall to be the agents responsible for
applying pressure to your system, or the barostat. I don't know what
kind of strange behavior will result if you try to do both
simultaneously.)

Cheers

Andrew