# BN sheet not expanding, but boundaries line expanding in tensile test

Dear all lammps users,

I’m simulating a BN system with dimensions 5*5nm. Below is my input file and I’m using BNC.tersoff potential for this system.
I’m facing a problem, while doing tensile test, BN sheet is not expanding, while boundary lines are expanding.

I’ve tried the same input file for Graphene, (similar structure to BN ), It is working fine.
I suspect that the problem in BN is having two different atoms of different mass.

Please throw some light on causes of this error.
I would be grateful to you all

Input file :

echo both
units metal
atom_style full

boundary p p p
newton on

dimension 3

pair_style tersoff
pair_coeff * * BNC.tersoff B N

neighbor 2.0 bin
neigh_modify delay 1

timestep 0.0005
fix 1 all npt temp 300.0 300.0 0.05 y 0 0 0.05
thermo 200
compute 1 all stress/atom NULL
compute 2 all pe/atom pair bond
compute 3 all reduce sum c_1[1] c_1[2] c_1[3]
thermo_style custom step temp pe ke etotal press lx ly lz
run 60000
unfix 1

fix 1 all nvt temp 300 300 0.05
fix 2 all ave/atom 1 1000 1000 c_1[1] c_1[2] c_1[3] c_2 fx fy fz
fix 3 all ave/time 1 1000 1000 c_thermo_press[2] file stress.all
dump 1 all custom 1000 dump.new.* id type x y z vx vy vz c_1[1] c_1[2] c_1[3] c_2 f_2[1] f_2[2] f_2[3] f_2[4] f_2[5] f_2[6] f_2[7]
dump 2 all xyz 1000 dump.graphene.*.xyz

variable tmp equal “lx”
variable L0 equal {tmp} print "Initial Length, L0: {L0}"
variable srate equal 1.0e9
variable srate1 equal “v_srate/1.0e12”
fix 4 all deform 1 x erate \${srate1} units box remap x
variable strain equal “(lx-v_L0)/v_L0”
variable p0 equal “v_strain”
variable p1 equal “-(pxx)/10000”
variable p2 equal “-(pyy)/10000”

fix 5 all print 1000 "{p0} {p1} \${p2} " file strain.txt screen no
thermo_style custom step temp v_strain v_p1 v_p2 press
run 600000
echo both

Khannan Krishna
Research scholar,
Department of Material science,
university of calicut.

Dear Axel/Steve,
I assume that you are busy, But really thankful to you if you enlighten the issues that causing only boundary lines to expand.

Thanking you,
Khannan Krishna.

This is the relevant command:
fix 4 all deform 1 x erate \${srate1} units box remap x

which should be adjusting the positions of all atoms due to remap
x as the box size grows.

So what does this mean:

BN sheet is not expanding, while boundary lines are expanding.

Are you visualizing, are you looking at atom coords in a dump file?

Are you pulling too fast? If the system doesn’t want to be in a larger
periodic box, then it will “tear” and some atoms (possibly at the boundary)
will move and some will not.

Steve

Thank you, It is because of large simulation box size.