[lammps-users] modeling diamond indentation of copper

Dear lammps-users,

I was hoping someone might be able to point me in the right direction for a problem I have...

I am trying to model nano cutting of copper with a diamond indenter. At the moment, the copper atoms explode outwards when the simulation begins. I am using a copper setup used by most - a fixed outer layer (bottom and left), just inside this a fixed 300K thermo layer, then the moveable copper atoms.

The temperature and pressure may be the reason as they are fluctuating quite a bit, but I don't understand why...

Can anyone see a reason why this might be occurring?

Below is the input script:

units metal
dimension 3
boundary s s s

# Create Simulation Box

lattice fcc 3.3 orient y 1 -1 0 orient x 1 1 1 orient z 1 1 -2
region box block 0 30 0 15 0 16
create_box 2 box

# Create bench

region cop block 0 16 0 8 0 16
lattice fcc 3.3 orient x 1 1 1 orient y 1 -1 0 orient z 1 1 -2
create_atoms 1 region cop

# Create indenter

lattice none
lattice diamond 3.3 orient x 1 1 1 orient y 1 -1 0 orient z 1 1 -2
region cyl cylinder z 20 10 2 7 9 # (x=20,y=10,r=2,zlo=7,zhi=9)
region front block 18 18.5 10 14 7 9
region back prism 20.414 21.414 8.586 15 7 9 10 0 0
region pin union 3 cyl front back

create_atoms 2 region pin

region bott block INF INF INF 1 INF INF
region left block INF 1 INF INF INF INF
region b block 1 16 1 2 INF INF
region l block 1 2 1 8 INF INF
group bott region bott # bottom layer
group left region left # left layer
group b region b
group l region l
group cu region cop
group pin region pin # indenter
group mob subtract cu bott left # moveable copper atoms
group exc union left bott pin # atoms that won't move
group therm union b l # thermo atoms

mass 1 63.546
mass 2 12.011

pair_style hybrid morse 4.5 eam
pair_coeff * * eam /Users/tim/Lammps/lammps-6Dec08/potentials/Cu_u3.eam
pair_coeff 1 2 morse 4.5 0.087 5.14 2.05

neighbor 2.0 bin
neigh_modify delay 5 exclude group exc exc

dump 1 all xyz 1000 dump.sim8

fix 12 therm temp/rescale 1.0 300 300 0.1 1.0
fix 5 pin nve/noforce
fix 10 mob nve

timestep 0.005
thermo 10
thermo_modify lost warn
thermo_style custom step temp press etotal pe

run 2000

velocity pin set -1 0 0 sum yes units box # give the indenter velocity 1A/ps in the x direction
run 8000

I have been through the manual trying to understand what could be happening, but have so far been unable to find it out.. If possible, thankyou for any advice.

Tim

Atoms "explode" b/c something is pushing on them
hard. Could be overlap with other atoms, or a region
that is too dense that has no boundary condition.

Steve